Which of the following acts as both reducing and oxidising agents?

In $( \text{a} ) ,( \text{b} )$ and $\left(\text{d}\right);$ Sulphur, nitrogen and Mn are in their maximum oxidation state, so they can only be reduced and thus can act only as oxidising agent.

$( \text{c} )$ only {\text{H}_{2}{\text{O}_{2} will act as both.

(i) In case of oxidising agent

{\text{H}_{2}{\text{O}_{2} itself get reduced:

e.g.,

(ii) In case of reducing agent {\text{H}_{2}{\text{O}_{2} itself get oxidise