Complex (I) has the central metal ion as Fe2+ with strong field ligands. Configuration of Fe2+=[Ar]3d6 Strong field ligands will pair up all the electrons and hence the magnetic moment will be zero. Complex(II) has the central metal ion as Cr2+ with weak field ligands. Configuration of Cr2+=[Ar]3d4 As weak field ligands are present, pairing does not take place. There will be 4 unpaired electrons and hence the magnetic moment =√24 B.M. Complex (III) has the central metal ion as Co2+ with weak field ligands. Configuration of Co2+=[Ar]3d7 As weak field ligands are present no pairing can occur. There will be 3 unpaired electrons and hence the magnetic moment =√15 B.M. Complex (IV) has the central metal ion as Fe3+ with strong field ligands. Configuration of Fe3+=[Ar]3d5 Strong field ligands will pair up the electrons but as we have a [Ar] 3d5 configuration, one electron will remain unpaired and hence the magnetic moment will be √3 B.M.