d²sp²
d²sp³
dsp²
sp³
[Ni(CN)4]²⁻ is an square planar complex with dsp² hybridization.
The outer electronic configuration of Ni²+ ion is 3d⁸4s⁰. One 3d, one 4s and two 4p orbitals undergo dsp² hybridization four equivalent dsp² hybrid orbitals which are occupied by 4 electron pairs of 4 cyanide ligands. This forms [Ni(CN)4]²⁻. Hence, option C is correct.