Fe2+ electronic configuration is [Ar]3d6 Since CN is strong field ligand d electrons are paired . In Ni(CO)4 O. S. of Ni is zero electronic configuration is [Ar]3d84s2.In presence of CO it is [Ar]3d104s0, electrons are paired. Electronic configuration of Ni2+[Ar]3d84s0, due to CN− ligand all electrons are paired. Co3+ is [Ar]3d6 since F is weak ligand hence paramagnetic.