For M²⁺/M and M³⁺/M²⁺ systems the E° values for some metals are as follows:
Cr²⁺/Cr – 0.9 V Cr³⁺/Cr²⁺ – 0.4 V
Mn²⁺/Mn – 1.2 V Mn³⁺/Mn²⁺ + 1.5 V
Fe²⁺/Fe – 0.4 V Fe³⁺/Fe²⁺ + 0.8 V
Use this data to comment upon
(a)    The stability of Fe³⁺ in acid solution as compared to that of Cr³⁺ or Mn³⁺ and
(b)    In case with which iron can be oxidised as compared to the similar process for either chromium or manganese metal

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.