Which one of the following elements is unable to form $\mathrm{M}{\mathrm{F}}_6^{3-} \mathrm{ion}?$

Boron does not have vacant d -orbitals in its valence shell, so it cannot extend its covalency beyond 4. i.e., 'B' cannot form the ions like $M{\mathrm{F}}_6^{3-}$