Explanation

(c)
Spin only magnetic moments depend upon the number of unpaired electrons, more the number of unpaired electron, greater will be the spin only magnetic moment.
_{25}Mn=$1s^{2},\ 2s^{2}2p^{6},\ 3s^{2}3p^{6}3d^{5}4s^{2}$
${Mn}^{2 +} = 1s^{2},\ 2s^{2}\ 2p^{6},3s^{2}\ 3p^{6}\ 3d^{5},4s^{0}$
[table] $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ [/table]
Number of unpaired electrons=5
_{24}Cr=$1s^{2},2s^{2}\ 2p^{6},3s^{2}\ 3p^{6}3d^{5},4s^{1}$
${Cr}^{2 +} = 1s^{2},2s^{2}2p^{6},\ 3s^{2}3p^{6}\ 3d^{4},4s^{0}$
[table] $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ | [/table]
Number of unpaired electron=4
23V=$1s^{2},2s^{2}\ 2p^{6},3s^{2}\ 3p^{6}\ 3d^{3},4s^{2}$
$V^{2 +} = 1s^{2},2s^{2}\ 2p^{6},3s^{2}\ 3p^{6}\ 3d^{3},4s^{0}$
[table] $\upharpoonleft$ | $\upharpoonleft$ | $\upharpoonleft$ | | [/table]
Number of unpaired electrons =3
So, the correct order of spin only magnetic moment is
${Mn}^{2 +}\ > {Cr}^{2 +} > \ V^{2 +}$