Identify whether the following reactions act as oxidation or reduction processes:
$(i)$ $FeSO_4 + Mg \to MgSO_4 + Fe$
$(ii)$ $Cu + 4HNO_3 \to Cu(NO_3)_2 + 2NO_2 + 2H_2O$
$(iii)$ $H_2S + Cl_2 \to S + 2HCl$

(N/A) To determine if a reaction involves oxidation or reduction,we look at the change in oxidation states:
$(i)$ $FeSO_4 + Mg \to MgSO_4 + Fe$: Here,$Mg$ (oxidation state $0$) is oxidized to $Mg^{2+}$ (in $MgSO_4$),and $Fe^{2+}$ (in $FeSO_4$) is reduced to $Fe$ $(0)$. This is a redox reaction.
$(ii)$ $Cu + 4HNO_3 \to Cu(NO_3)_2 + 2NO_2 + 2H_2O$: Here,$Cu$ $(0)$ is oxidized to $Cu^{2+}$ $(+2)$,and $N$ ($+5$ in $HNO_3$) is reduced to $N$ ($+4$ in $NO_2$). This is a redox reaction.
$(iii)$ $H_2S + Cl_2 \to S + 2HCl$: Here,$S$ ($-2$ in $H_2S$) is oxidized to $S$ $(0)$,and $Cl$ ($0$ in $Cl_2$) is reduced to $Cl$ ($-1$ in $HCl$). This is a redox reaction.
All three reactions are redox reactions involving both oxidation and reduction.