In [Co(NH3)6]3+, Co is in +3 oxidation state with the configuration 3d6. In the presence of NH3 a strong field ligand, the 3d electrons undergo pairing and leaving two d-orbitals empty. Hence, the hybridisation is d2sp3 which forms an inner orbital octahedral complex. The complex is diamagnetic as it has no unpaired electrons.

So, the correct option is C