Which of the following is not an example of redox reaction?

Option (iv) is not a redox reaction as the oxidation number of Ba and S in the reaction equation is +2 and +4 respectively in both reactant and product side is same. As redox reaction is only related to the change in oxidation states of reactants the selected option does not show reduction and oxidation.

Options (i)& (ii) are example of a displacement redox reaction

Option (iii) is a form of combination redox reaction.