Which of the following groups of molecules act as both an oxidising agent and a reducing agent?

When ammonia is passed over heated copper oxide,metallic copper is obtained. This reaction shows that ammonia is a.......

In the preparation of chlorine from $HCl,$ $MnO_2$ acts as

The strongest reducing agent among the given options is:

Which among $KI, FeSO_4, K_2MnO_4, KMnO_4$ cannot be oxidised by $O_3$?

Consider the reactions:
$(a)$ $H_3PO_{2(aq)} + 4AgNO_{3(aq)} + 2H_2O_{(l)} \rightarrow H_3PO_{4(aq)} + 4Ag_{(s)} + 4HNO_{3(aq)}$
$(b)$ $H_3PO_{2(aq)} + 2CuSO_{4(aq)} + 2H_2O_{(l)} \rightarrow H_3PO_{4(aq)} + 2Cu_{(s)} + 2H_2SO_{4(aq)}$
$(c)$ $C_6H_5CHO_{(l)} + 2[Ag(NH_3)_2]^{+}_{(aq)} + 3OH^{-}_{(aq)}$ $\rightarrow C_6H_5COO^{-}_{(aq)} + 2Ag_{(s)} + 4NH_{3(aq)} + 2H_2O_{(l)}$
$(d)$ $C_6H_5CHO_{(l)} + 2Cu^{2+}_{(aq)} + 5OH^{-}_{(aq)} \rightarrow$ No change observed.
What inference do you draw about the behaviour of $Ag^{+}$ and $Cu^{2+}$ from these reactions?