While ${\mathrm{Fe}}^{3+}$is stable, ${\mathrm{Mn}}^{3+}$is not stable in acid solution because 

While ${\mathrm{Fe}}^{3+}$ is stable, ${\mathrm{Mn}}^{3+}$ is not stable in acid solution because
${\mathrm{Mn}}^{3+}\text{ oxidises }{\mathrm{H}}_2\mathrm{O}\text{ to }{\mathrm{O}}_2\text{. }$

Reaction of ${\mathrm{Mn}}^{3+}$ with ${\mathrm{H}}_2\mathrm{O}$ is spontaneous as $\mathrm{E}°$ value of ${\mathrm{Mn}}^{3+}+{\mathrm{e}}^{-}\to$
${\mathrm{Mn}}^{2+}\mathrm{E}°=1.50$
is higher than the $\mathrm{E}°$ value of ${\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_{2}O E°=1.23$.

Therefore, the correct option is (D).