What is a redox couple?

In a solution of $AgNO_3$,if $Cu$ is added,the solution becomes blue due to:

In the reactions given below,identify the species undergoing oxidation and reduction:
$(i)$ $H_{2}S_{(g)} + Cl_{2(g)} \rightarrow 2HCl_{(g)} + S_{(s)}$
$(ii)$ $3Fe_{3}O_{4(s)} + 8Al_{(s)} \rightarrow 9Fe_{(s)} + 4Al_{2}O_{3(s)}$
$(iii)$ $2Na_{(s)} + H_{2(g)} \rightarrow 2NaH_{(s)}$

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 $H_2S + NO_2 \to H_2O + NO + S$,$H_2S$ is

Which of the following elements exhibits the usual tendency to undergo reduction?