The correct option is D NaCl
Since all the compounds have the same anion, i.e., Cl−, the size of cations will be considered.
Melting point depends upon the lattice energy. Greater the lattice energy, greater is the melting point. Smaller the size of the ions, greater the lattice energy.
Down the group, size increases, i.e., Na+<K+<Rb+<Cs+
Hence, among all NaCl has the highest melting point as it has the highest lattice energy.