The correct option is B [Fe(H2O)6]2+
This is an interesting question. What does paramagnetism has anyting to do with what we have learned in coordination compounds! Remember that for paramagnetism to exist, you need to have unparied electrons in individual subshells. I think you may be able to catch what I am talking about by now.
This has to do with VBT! Remember, strong field ligands force pairing of electrons and weak field ligands do not. So the compounds with weak field ligands have the highest chance of showing maximum paramagnetism.
All these options have water as the ligand which is a weak field ligand so all we need to do is use the Hund's rule to fill up electrons in d subshells, the one with the most unpaired electrons would be the most paramagnetic.
Looking at these options, Fe2+ would have a 3d6 configuration. Hence, in option (b), there is no pairing and there are 4 unpaired electrons.
Cr (III) has 3 electrons, Zn (II) has no unpaired electrons (3d10 configuration), and copper (II) has just one unpaired electron (3d9 configuration)