Relative to $\mathrm{NaCl}$ (m.p. $801^{\circ }\mathrm{C}$), which of the following have a lower melting point mainly because of significant polarisation (covalent character)?

$\mathrm{CuCl}$ has an $18$-electron cation, while $\mathrm{Be}{}^{2+}$ and $\mathrm{Al}{}^{3+}$ are small and highly charged, so $\mathrm{CuCl}$, $\mathrm{BeCl}{}_2$ and $\mathrm{AlCl}{}_3$ are partly covalent and melt (or sublime) at lower temperatures. $\mathrm{MgF}{}_2$ has a doubly charged cation with the hardly polarisable $\mathrm{F}{}^{-}$, so it is ionic with a high lattice energy and a higher melting point (about $1260^{\circ }\mathrm{C}$).