Oxidation-reduction also known as redox reaction occurs only when there is change in oxidation state of atoms. An atom oxidation state changes when it loses or gains one or more electrons. In other words, redox reaction has two parts, reduced half in which atom gains the electron and oxidized half in which atom loses the electron. In given options, only Zn + S --> ZnS perform redox as atoms went from neutral oxidation state to gaining and losing 2 electrons. More on redox reactions in linked lecture.
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