We know that higher the reduction potential of a specie, there is larger tendency for its reduction to take place. Let us represent the reduction potentials i.e., E° values of three M3+/M2+ systems on number line.

Cr3+ /Cr2+ =-0.4V
Fe3+/Fe2+ =0.8V
Mn3+/Mn2+ =1.5V

Fom values of reduction potentials, it is clear that the stability of Fe3+ in acid solution is more than that Cr3+ but less than that of Mn3+.

b) Mn2+/Mn =-1.2V
Cr2+/Cr =-0.9V
Fe2+/Fe =-0.4V

The lower the reduction potential of a species, its oxidation will take place more readily.
∴ Mn is oxidised most readily to Mn2+ and Fe is oxidised least readily among given metals Mn, Cr and Fe.We know that higher the reduction potential of a specie, there is larger tendency for its reduction to take place. Let us represent the reduction potentials i.e