The pair of metal ions that can give a spin only magnetic moment of 3.9 BM for the complex [M(H2O)6]Cl2, is:

Spin only magnetic moment given as μ=√n(n+2) where n=number of unpaired electron μ=√n(n+2)=3.9 ⇒n≈3 So, there are 3 unpaired electrons which is possible only in the compounds shown in the figure, V+2 and Co+2. Others won't have 3 unpaired electrons- Cr+2 ⇒3d4 ⇒4 unpaired electrons Mn+2 ⇒3d5 ⇒5 unpaired electrons Fe+2 ⇒3d6 ⇒4 unpaired electrons Hence, the correct is B.