A.Cu+Zn2+→Cu2++Zn
Here, Cu is oxidized to Cu2+ and Zn2+ is reduced to Zn.
Hence, the redox couples are Cu2+/Cu and Zn2+/Zn.
B.Mg+Fe2+→Mg2++Fe
Here Mg is oxidized to Mg2+ and Fe2+
Hence, the redox couples are Mg2+/Mg and Fe2+/Fe.
C.Br2+2Cl−→Cl2+2Br−
Here Cl− is oxidized to Clz and Br2 is reduced to Br−
Hence, the redox couples are Br2/Br− and Cl2/Cl−
D.Fe+Cd2+→Cd+Fe2+
Here, Fe is oxidized to Fe2+ and Cd2+ is reduced to Cd.
Hence, the redox couples are Fe2+/Fe and Cd2+/Cd.