In case of strong field ligand containing complexes, electrons get pair up against the Hund's rule to get required number of hybrid orbitals. Among the given complexes' ammonia is a strong-filed ligand and other ligands are weak filed ligands. [Co(NH3)6]3+ has d2sp3 (inner orbital complex) with zero unpaired electrons. Hence, it is octahedral with diamagnetic character and low spin complex. [CoF6]3– and [CoCl6]3– are octahedral but having unpaired electrons.[NiCl4]2− is not octahedral.