${\left[{\mathrm{AlF}}_6\right]}^{3-}$ exists, while ${\left[{\mathrm{BF}}_6\right]}^{3-}$ does not. Choose the most appropriate reason.

All the options except (c) are correct but the most appropriate reason is that boron cannot expand its valence shell and its maximum valency is limited to 4. While Al can expand its valence shell and hence can form ${\left[{\mathrm{AlF}}_6\right]}^{3-}$.