| Ion | Electronic configuration | No. of unpaired electron | |
| 1 | Cr2+ | 3d4 | 4 |
| 2 | V2+ | 3d3 | 3 |
| 3 | Fe2+ | 3d6 | 4 |
| 4 | Ni2+ | 3d8 | 2 |
Magnetic moment (spin only)is directly proportional to the number of unpaired electrons.
Hence, less is the number of unpaired electrons less is the magnetic moment (spin only).
As Ni^{2+ }has the least number of unpaired electrons it has the least magnetic moment (spin only) among the given options.