Lattice energy is defined as the amount of energy required to separate one mole of solid ionic compound into its gaseous ions. Evidently greater the lattice energy, higher is the melting point of the alkali metals halide and lower is its solubility in water.
For the same halide ion, the melting point of lithium halides are lower than those of the corresponding sodium halides and thereafter they decrease as we move down the group from Na to Cs.
The low melting point of \(\mathrm{LiCl}(887 \mathrm{~K})\) as compared to \(\mathrm{NaCl}\) is probably because \(\mathrm{LiCl}\) is covalent in nature and \(\mathrm{NaCl}\) is ionic.
Hence the correct order is \(\mathrm{NaCl} > \mathrm{KCl} > \mathrm{CsCl} > \mathrm{LiCl}\).