Correct option is (a) i.e. LiF>LiCl;MgO>NaCl. Melting point is directly proportional to lattice energy. Lattice energy is the energy required to separate a mole of an ionic solid into gaseous ions. It depends upon charge of ions and size of ions. M.P∝L.E∝
Charge
Size
Li→+1Li→+1 F=−1Cl→−1 Both LiF and LiCl having same charge, so melting point will depend on size. Larger the size of anion, lesser the lattice energy and hence, melting point order is LiF>LiCl. Similarly, MgONaCl Mg→2+Na→1+ O→2−Cl→1− MgO having +2 charge which is greater than NaCl(+1) charge. So, greater the charge on the ions greater will be lattice energy and hence, melting point order is MgO>NaCl.