Among the given option (a) is a diamagnetic complex. (a) [Co(NH3)6]3+⇒Co(27)⇒Co3+=3d64s0. Thus, has no unpaired electrons and is a diamagnetic complex. (b) [NiCl4]2−⇒Ni(28)⇒Ni2+=3d84s0 Thus, has two unpaired electrons is paramagnetic. (c) [CuCl4]2−⇒Cu(29)⇒Cu2+=3d94s0 Thus, has one unpaired electrons and is paramagnetic. (d) [Fe(H2O)6]3+⇒Fe(26)⇒Fe3+=3d54s0 Thus, has five unpaired electrons and is paramagnetic.