The E^ value for the Mn^ 3+ / Mn^ 2+ couple is more positive than...

$\mathrm{E}_{\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}}^{\circ}>\mathrm{E}_{\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}}^{\circ} \text{ or } \mathrm{E}_{\mathrm{Fe}^{3+} / \mathrm{Fe}^{2+}}^{\circ}$ 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.