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Question 1. Which property of a blockchain ensures that once data is written it cannot be altered without detection? A) Transparency B) Decentralization C) Immutability D) Consensus Answer: C Explanation: Immutability means that data, once recorded in a block, cannot be changed without breaking the hash chain, making tampering evident. Question 2. In the UTXO model used by Bitcoin, a transaction’s inputs are: A) Account balances of the sender B) References to previous unspent transaction outputs C) Smart contract states D) Merkle proofs of inclusion Answer: B Explanation: Each input points to a prior unspent output, which is then consumed, ensuring double-spending protection. Question 3. Which consensus algorithm is most vulnerable to a 51 % attack? A) Proof-of-Stake B) Proof-of-Work C) Practical Byzantine Fault Tolerance D) Proof-of-Authority Answer: B
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Explanation: In PoW, if an entity controls >50 % of the hashing power, it can rewrite history and double-spend. Question 4. A smart contract written in Solidity that should never change its state after deployment is best implemented using: A) A library B) An abstract contract C) An immutable variable D) A proxy pattern Answer: A Explanation: Libraries are deployed once and contain only pure or view functions; they cannot hold state, making them effectively immutable. Question 5. Which Ethereum account type can sign transactions? A) Contract account B) Externally owned account (EOA) C) DAO account D) Multisig account only Answer: B Explanation: Only EOAs possess private keys and can produce signatures for transaction authentication. Question 6. In Solidity, which visibility modifier restricts a function to be called only from within the same contract or derived contracts? A) public B) private C) internal D) external
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B) Placing state changes before external calls (checks-effects-interactions) C) Declaring the function as pure D) Setting a high gas limit Answer: B Explanation: Updating state before making external calls prevents the attacker from re-entering the function with stale state. Question 10. Which cryptographic hash function is used by Bitcoin for block hashing? A) SHA- 1 B) SHA- 256 C) Keccak- 256 D) Blake2b Answer: B Explanation: Bitcoin employs SHA-256 for proof-of-work and block header hashing. Question 11. In Hyperledger Fabric, the component responsible for ordering transactions into blocks is called: A) Peer B) Orderer C) Endorser D) Chaincode Answer: B Explanation: The Orderer service implements the consensus protocol and assembles endorsed transactions into ordered blocks.
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Question 12. Which Solidity data type is appropriate for storing a dynamic list of addresses? A) address[5] B) address[] C) mapping(address => uint) D) bytes32 Answer: B Explanation: address[] declares a dynamic array that can grow or shrink at runtime. Question 13. A Merkle tree in a blockchain is primarily used to: A) Encrypt transaction data B) Enable fast verification of transaction inclusion C) Store smart contract code D) Generate consensus votes Answer: B Explanation: Merkle proofs allow a node to confirm a transaction’s presence in a block with minimal data. Question 14. Which Layer-2 solution relies on fraud proofs to ensure correctness? A) ZK-Rollup B) Optimistic Rollup C) State Channels D) Plasma Answer: B Explanation: Optimistic Rollups assume transactions are valid but allow anyone to submit fraud proofs within a challenge window.
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Explanation: Oracles bridge off-chain data to on-chain contracts; Chainlink is a widely adopted solution. Question 18. In the context of blockchain trilemma, which combination is hardest to achieve simultaneously? A) Decentralization, security, and scalability B) Transparency, immutability, and speed C) Privacy, speed, and cost D) Governance, speed, and consensus Answer: A Explanation: The trilemma states that improving two of decentralization, security, and scalability often degrades the third. Question 19. Which of the following best describes a “cold wallet”? A) A wallet that stores private keys on a hardware device offline B) A wallet that is always connected to the internet C) A wallet that uses a smart contract for key management D) A wallet that requires a password for every transaction Answer: A Explanation: Cold wallets keep private keys offline (e.g., hardware wallets, paper wallets) to reduce exposure to attacks. Question 20. The delegatecall opcode in Solidity is used to: A) Transfer Ether to another address B) Execute code from another contract while preserving the caller’s storage context C) Create a new contract D) Self-destruct the contract
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Answer: B Explanation: delegatecall runs the code of the target contract but uses the storage, msg.sender, and msg.value of the calling contract. Question 21. In an ERC-721 token, the function that returns the URI of a token’s metadata is: A) tokenURI(uint256 tokenId) B) metadata(uint256 tokenId) C) uri(uint256 tokenId) D) getTokenData(uint256 tokenId) Answer: A Explanation: tokenURI is defined in the ERC-721 standard to retrieve a token’s metadata location. Question 22. Which statement about Proof-of-Authority (PoA) consensus is true? A) Validators are chosen based on the amount of stake they hold. B) Validators are pre-approved identities, often known entities. C) Mining rewards are distributed to all network participants. D) It uses a random selection of nodes for each block. Answer: B Explanation: PoA relies on a set of authorized validators whose identities are known and trusted. Question 23. In a Hyperledger Fabric channel, private data collections allow: A) All network participants to see every transaction.
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Question 26. Which of the following is NOT a typical attack vector for smart contracts? A) Reentrancy B) Integer overflow/underflow C) Sybil attack on the consensus layer D) Timestamp dependence Answer: C Explanation: Sybil attacks target the network’s consensus, not the internal logic of a smart contract. Question 27. In Ethereum, the fallback function is triggered when: A) A contract receives Ether with no matching function signature B) A contract is deployed C) A transaction is mined in a block D) A contract calls another contract’s receive function Answer: A Explanation: The fallback function (or receive in newer versions) runs when no other function matches the calldata. Question 28. Which tool is primarily used for writing, compiling, and testing Solidity contracts in a browser? A) Hardhat B) Truffle C) Remix IDE D) Ganache Answer: C Explanation: Remix is a web-based IDE that supports Solidity compilation, debugging, and deployment.
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Question 29. The primary purpose of a “nonce” in an Ethereum transaction is to: A) Encrypt the transaction data B) Prevent replay attacks by ensuring uniqueness C) Determine the gas price D) Sign the transaction with a private key Answer: B Explanation: Each transaction from an address must have a unique nonce, which stops duplicate or replayed transactions. Question 30. Which of the following best describes a “state channel” solution? A) Off-chain execution of many transactions with only opening and closing on-chain B) A sidechain that runs its own consensus algorithm C) A sharding technique for scaling block size D) A method for storing large files off-chain Answer: A Explanation: State channels allow participants to transact off-chain, committing only the final state to the main chain. Question 31. In Solidity, mapping(address => uint) balances; is stored: A) In the contract’s code segment B) In a separate contract called “MappingStore” C) In storage, with each key hashed to a slot D) In memory only during execution Answer: C
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D) Built-in consensus algorithm Answer: B Explanation: IPFS stores data by its hash, providing decentralized, tamper-evident file storage. Question 35. A “gas limit” set too low for a transaction will: A) Increase the transaction’s priority in the mempool B) Cause the transaction to be dropped or revert due to out-of-gas C) Reduce the transaction fee paid to miners D) Allow the transaction to execute for free Answer: B Explanation: If execution requires more gas than the limit, the EVM aborts and reverts the state changes. Question 36. Which of the following is true about a “soft fork” in blockchain terminology? A) It creates a new cryptocurrency with a different ledger. B) It is backward-compatible; non-upgraded nodes continue to operate. C) It always results in a chain split. D) It changes the block size limit to a smaller value. Answer: B Explanation: Soft forks introduce stricter rules that older clients can still recognize as valid. Question 37. In a Solidity contract, the onlyOwner modifier typically: A) Allows any address to call the function B) Restricts the function to the contract’s deployer address
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C) Prevents the function from being called more than once D) Enables reentrancy protection Answer: B Explanation: onlyOwner checks that msg.sender matches the stored owner address, limiting access. Question 38. Which of the following is a characteristic of a “public” blockchain? A) Only approved entities can read data. B) Nodes must obtain permission from a central authority. C) Anyone can join the network and validate transactions. D) Transactions are always private. Answer: C Explanation: Public blockchains are permissionless; participation in consensus is open to anyone. Question 39. The event keyword in Solidity is used to: A) Store persistent data on the blockchain B) Emit logs that can be indexed off-chain for UI updates C) Execute code on a remote server D) Perform arithmetic operations with overflow checks Answer: B Explanation: Events generate logs that are not stored in contract storage but can be listened to by off-chain applications. Question 40. In the context of blockchain, “finality” refers to: A) The moment a block is mined
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A) Reducing gas consumption B) Preventing integer overflow and underflow bugs C) Enabling multi-signature wallets D) Providing encryption utilities Answer: B Explanation: SafeMath adds checks that revert on overflow/underflow, which Solidity versions prior to 0.8 did not enforce. Question 44. Which of the following best describes a “delegated proof-of-stake” (DPoS) system? A) Validators are randomly selected based on stake size. B) Token holders elect a small number of delegates to produce blocks. C) Mining is performed using GPU hardware. D) All nodes must run the full transaction history. Answer: B Explanation: DPoS relies on elected delegates (or witnesses) who are responsible for block production. Question 45. In a Hyperledger Fabric network, the “Chaincode” is analogous to: A) A mining algorithm B) A smart contract that runs on peers C) The consensus protocol D) The network’s ordering service Answer: B Explanation: Chaincode implements business logic and is executed by endorsing peers.
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Question 46. Which of the following is a primary function of a “validator” in a PoS blockchain? A) Solve cryptographic puzzles B) Produce blocks proportional to their stake and receive rewards C) Store the full transaction history only D) Broadcast transaction fees to miners Answer: B Explanation: Validators lock up stake, propose/attest to blocks, and earn rewards according to their stake. Question 47. The delegatecall vulnerability often leads to: A) Loss of Ether due to reentrancy B) Unexpected changes to the calling contract’s storage C) Increased gas consumption without effect D) Immutable contract code Answer: B Explanation: Since delegatecall uses the caller’s storage, a malicious callee can overwrite critical variables. Question 48. Which Solidity version introduced custom errors that reduce gas compared to require() strings? A) 0.6. B) 0.7. C) 0.8. D) 0.9. Answer: C Explanation: Solidity 0.8.4 added error definitions that can be reverted with a selector, saving gas.
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Explanation: Miners vary the nonce to produce different hash outputs until a valid proof-of-work is found. Question 52. Which of the following is NOT a valid Solidity state mutability keyword? A) view B) pure C) constant (as of Solidity 0.8) D) payable Answer: C Explanation: constant was deprecated for functions; it remains for variables but not as a function mutability specifier. Question 53. In the context of token standards, ERC-1155 differs from ERC-20 and ERC-721 because it: A) Supports both fungible and non-fungible tokens in a single contract B) Is only used for stablecoins C) Requires a separate contract per token type D) Does not allow batch transfers Answer: A Explanation: ERC-1155 enables mixed token types and batch operations, improving efficiency. Question 54. Which of the following best explains a “51 % attack” in a PoW blockchain? A) Controlling 51 % of the network’s total stake B) Controlling 51 % of the total transaction volume C) Gaining majority hash power to rewrite recent blocks and double-spend
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D) Owning 51 % of all cryptocurrency supply Answer: C Explanation: With >50 % of hashing power, an attacker can outpace honest miners, creating alternate chains. Question 55. The term “gas” in Ethereum is analogous to: A) Bandwidth in networking B) CPU cycles in traditional computing C) Storage capacity in cloud services D) Mining difficulty Answer: B Explanation: Gas measures computational effort, similar to CPU cycles, and determines the cost of executing operations. Question 56. Which of the following is a typical use case for a “state channel”? A) Storing large files on-chain B) High-frequency micro-transactions between two parties C) Deploying a DAO governance contract D) Running a public token sale Answer: B Explanation: State channels excel at rapid, low-fee exchanges between participants before settling on-chain. Question 57. In Solidity, the selfdestruct function: A) Pauses the contract indefinitely B) Deletes the contract code and sends remaining Ether to a specified address
