In which process does the nitrogen undergo oxidation?

Identify the redox couples involved in the following reactions:
$(a) \ Cu + Zn^{+2} \to Cu^{+2} + Zn$
$(b) \ Mg + Fe^{+2} \to Mg^{+2} + Fe$
$(c) \ Br_{2} + 2Cl^{-} \to Cl_{2} + 2Br^{-}$
$(d) \ Fe + Cd^{+2} \to Cd + Fe^{+2}$

In the reaction $3Mg + N_2 \to Mg_3N_2$,which of the following is correct?

When ammonia is heated with cupric oxide,the ammonia molecule $.......$

In the reaction,$2KMnO_4 + 16HCl \rightarrow 5Cl_2 + 2MnCl_2 + 2KCl + 8H_2O$,the reduction product is

$H_2S$ reacts with halogens,the halogens