The electronic configurations of the ions are: 1. $\mathrm{Mn^{2+}} (Z=25): [Ar] 3d^5$. Number of unpaired electrons = 5. 2. $\mathrm{Fe^{2+}} (Z=26): [Ar] 3d^6$. Number of unpaired electrons = 4. 3. $\mathrm{Cr^{3+}} (Z=24): [Ar] 3d^3$. Number of unpaired electrons = 3. Comparing the number of unpaired electrons: $5 > 4 > 3$. Therefore, the correct order is $\mathrm{Mn^{2+} > Fe^{2+} > Cr^{3+}}$. $$ \begin{array}{c}\begin{array}{|l|l|l|l|l|}\hline \upharpoonleft & \upharpoonleft & \upharpoonleft & \upharpoonleft & \upharpoonleft \\\hline\end{array}\\ \mathrm {3d^5}\end{array} $$ $$ \begin{array}{c}\begin{array}{|l|l|l|l|l|}\hline ~~~\upharpoonleft \downharpoonright~& \upharpoonleft & \upharpoonleft & \upharpoonleft & \upharpoonleft \\\hline\end{array}\\ \mathrm {3d^6}\end{array} $$ $$ \begin{array}{c}\begin{array}{|l|l|l|l|l|}\hline \upharpoonleft & \upharpoonleft & \upharpoonleft & ~ & ~\\\hline\end{array}\\ \mathrm {3d^3}\end{array} $$