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

Boron ($\text{B}$) belongs to the second period and has only $s$ and $p$ orbitals available for bonding ($2s,2p$). It lacks vacant $d$ orbitals, which limits its maximum covalency to $4$. Therefore, it cannot expand its octet to form six-coordinated species like ${\text{BF}}_6^{3-}$. Elements like $\text{Al, Ga,}$ and $\text{In}$ have vacant $d$ orbitals and can form octahedral complexes.