The octahedral diamagnetic low spin complex among the following is

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. ${\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}$ has $\mathrm{d}{ }^2{\mathrm{sp}}^3$ (inner orbital complex) with zero unpaired electrons. Hence, it is octahedral with diamagnetic character and low spin complex. ${\left[{\mathrm{CoF}}_6\right]}^{3–}$ and ${\left[{\mathrm{CoCl}}_6\right]}^{3–}$ are octahedral but having unpaired electrons.${\left[{\mathrm{NiCl}}_4\right]}^{2-}$ is not octahedral.