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. 25Mn=1s2,2s22p6,3s23p63d5,4s2 Mn2+=1s2,2s22p63s23p63d5,4s0
↑
↑
↑
↑
↑
Number of unpaired electrons =5 24Cr=1s2,2s22p6,3s23p63d5,4s1 Cr2+=1s2,2s22p6,3s23p63d4,4s0
↑
↑
↑
↑
Number of unpaired electrons =4 23V=1s2,2s22p6,3s23p63d3,4s2 V2+=1s2,2s22p6,3s23p63d3,4s0
↑
↑
↑
Number of unpaired electrons =3 So, the correct order of spin only magnetic moment is Mn2+>Cr2+>V2+