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

Which of the following substances can act as both an oxidizing agent and a reducing agent?

Which one of the following cannot function as an oxidising agent?

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

Which of the following oxides cannot act as a reducing agent?

When heated in air,brown copper powder turns black. This black powder would turn brown again when heated with: