Paramagnetism ∝ Number of unpaired electrons
In [Cr(H2O)6]3+, Cr(III) has 3d3 outer electronic configuration with 3 unpaired electrons.
In [Fe(H2O)6]2+, Fe(II) has 3d6 outer electronic configuration with 4 unpaired electrons.
In [Cu(H2O)6]2+, Cu(II) has 3d9 electronic configuration with 1 unpaired electron.
In [Zn(H2O)6]2+, Zn(II) has 3d10 electronic configuration with 0 unpaired electrons.
[Fe(H2O)6]2+ has the highest paramagnetism as it contains a maximum number of unpaired electrons.
Hence, option B is correct.