Among [NiCO4], [Ni(CN)4]2–, [NiCl4]2– species, the hybridisation states of the Ni atom are, respectively (At. no. of Ni = 28)

Outermost electronic configuration of [Ni(CO)4] complex is 3d8, 4s2 and CO is a strong ligand thus electrons get paired up and shows sp3 hybridisation. Outermost electronic configuration of [Ni(CN)4]–2 complex is 3d8 and CN– is a strong ligand thus electrons get paired up but one d orbital is empty and shows dsp2 hybridisation. Outermost electronic configuration of complex is 3d8, 13 COORDINATION COMPOUNDS 4s2 and Cl- is a weak ligand thus electrons does not paired up and shows sp3 hybridisation. Therefore So, option (b) is the correct answer.