The $\mathrm{E}^{\circ}$ value for the $\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}$ couple is more positive than…
The $\mathrm{E}^{\circ}$ value for the $\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}$ couple is more positive than that of $\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}$ or $\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}$ due to change of
$\mathrm{d}^5$ to $\mathrm{d}^2$ configuration
$\mathrm{d}^4$ to $\mathrm{d}^5$ configuration
$d^3$ to $d^5$ configuration
$d^5$ to $d^4$ configuration
Solution
$\mathrm{E}_{\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}}^{\circ}\gt\mathrm{E}_{\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}}^{\circ} \text { or } \mathrm{E}_{\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}}^{\circ}$
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 $\mathrm{Cr}^{3+}=[\mathrm{Ar}] 3 d^3$
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.