d
In \(\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}, \mathrm{Co}^{3+}\) ion is having \(3 \mathrm{~d}^6\) configuration.

Electronic configuration of \(\mathrm{Co}^{3+}\) \(Image\)

In presence of \(\mathrm{NH}_3\) ligand, pairing of electrons takes place and it becomes diamagnetic complex ion.

In presence of \(\mathrm{NH}_3\) ligand \(Image\)

\(\therefore \quad\left[\mathrm{Co}\left(\mathrm{NH}_3\right)_6\right]^{3+}\) is octahedral with \(\mathrm{d}^2 \mathrm{sp}^3\) hybridisation and it is diamagnetic in nature.

In case of \(\left[\mathrm{CoF}_6\right]^{3-}, \mathrm{Co}\) is in +3 oxidation state and it is having \(3 \mathrm{~d}^6\) configuration.

In presence of weak field \(\mathrm{F}^{-}\)ligand, pairing does not take place.

In presence of \(\mathrm{F}^{-}\)ligands \(Image\)

\(\therefore \quad \operatorname{In}\left[\mathrm{CoF}_6\right]^{3-}, \mathrm{Co}^{3+}\) is \(\mathrm{sp}^3 \mathrm{~d}^2\) hybridised with four unpaired electrons, so it is paramagnetic in nature.