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The $E^\circ$ value for the $Mn^{3+}/Mn^{2+}$ couple is more positive than that of $Cr^{3+}/Cr^{2+}$ or $Fe^{3+}/Fe^{2+}$ due to change of
A
$d^4$ to $d^5$ configuration
B
$d^3$ to $d^5$ configuration
C
$d^5$ to $d^4$ configuration
D
$d^5$ to $d^2$ configuration
Detailed Solution
$Mn^{3+}$ is $3d^4$ and $Mn^{2+}$ is $3d^5$.
The reduction $Mn^{3+} \rightarrow Mn^{2+}$ changes the configuration from $d^4$ to the extra-stable half-filled $d^5$, and the third ionisation enthalpy of Mn is very high. So $E^\circ$ for $Mn^{3+}/Mn^{2+}$ is highly positive.
$Cr^{3+}$ ($t_{2g}^3$) is more stable than $Cr^{2+}$, so $E^\circ$ for $Cr^{3+}/Cr^{2+}$ is negative.
So the reason is the change of $d^4$ to $d^5$ configuration.