An octahedral complex of Co³⁺ is diamagnetic. The hybridisation involved in the formation of the complex is:

If the octahedral complex is diamagnetic then it means all the electrons are paired up. The hybridisation involves two 3d orbitals, one 4s orbital and three 4p orbitals. Hence, the hybridisation of the octahedral compound is d²sp³. Option C is correct.