Compound having lowest melting point.

The covalent molecule beryllium chloride (${\mathrm{BeCl}}_2$) is not ionic. This is mostly because beryllium lacks cations due to its tiny size and high relative ionisation energy (900 kJ/mol). Instead, it just draws a bonding pair of electrons to itself.
Since beryllium chloride has a considerably lower boiling point than water, it must be covalent since it cannot contain ions. On the other hand, for such a tiny covalent molecule, the melting point is surprisingly high.
Hence correct option is A