Four successive members of the first row transition elements are listed below with atomic numbers. Which one of them is expected to have the highest ${\mathrm{E}}_{{\mathrm{M}}^{3+}/{\mathrm{M}}^{2+}}^0$ value ? 

SRP value normally increases from left to right in the period of d-block elements. Some SRP value are exceptionally higher due to stability of product ion.

$$\begin{gathered} {\mathrm{E}}_{{\mathrm{Mn}}^{3+}/\mathrm{Mn}}^{\mathrm{o}}=1.57\mathrm{V} \\ {\mathrm{E}}_{{\mathrm{Fe}}^{3+}/\mathrm{Fe} }^{\mathrm{o}}=0.77\mathrm{V}\hspace{0.17em} \\ {\mathrm{E}}_{{\mathrm{Co}}^{3+}/\mathrm{Co}}^{\mathrm{o}}= 1.97\mathrm{V} \\ {\mathrm{E}}_{{\mathrm{Cr}}^{3+}/\mathrm{Cr} }^{\mathrm{o}}= -0.41\mathrm{V} \end{gathered}$$

from their standard reduction potential it can be seen that Co has highest tendency to reduce into +2 oxidation state.