‌‌ Atomic number of ‌Fe(26)=[Ar]3d64s2 ‌‌‌‌‌Fe2+=[Ar]3d64s0
Number of unpaired electrons =4 For Co2+=[Ar]3d7 Number of unpaired electrons =3 It shows highest magnetic moment. As magnetic moment depends on number of unpaired electrons. For Cr3+=[Ar]3d3 ∴ Number of unpaired electrons =3 For Ni+=[Ar]3d8 ∴ Number of unpaired electrons =2