b
\(\mathrm{E}_{\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}}^0>\mathrm{E}_{\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}}^0 \text { or } \mathrm{E}_{\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}}^0\)

Electronic configuration of \(\mathrm{Mn}^{3+}=[\mathrm{Ar}] 3 d^4\)

Electronic configuration of \(\mathrm{Mn}^{2+}=[\mathrm{Ar}] 3 d^5\)

Electronic configuration of \(\mathrm{Cr}^{3+}=[\mathrm{Ar}] 3 d^\beta\)

Electronic configuration of \(\mathrm{Cr}^{2+}=[\mathrm{Ar}] 3 d^4\)

As \(\mathrm{Mn}^{3+}\) from \(d^4\) configuration goes to more stable \(d^5\) configuration (Half filled), due to more exchange energy in \(d^5\) configuration.