The correct set from the following in which both pairs are in correct order of melting point is:

The melting point of an ionic compound depends on the lattice energy, which in turn depends on the charges of the ions and the interionic distance. According to the Born-Landé equation, lattice energy is directly proportional to the product of ionic charges and inversely proportional to the sum of ionic radii.

Comparing LiF and LiCl: Both have the same cation ($$\text{Li}^+$$) and the same charges ($$+1$$ and $$-1$$). The fluoride ion ($$\text{F}^-$$) is smaller than the chloride ion ($$\text{Cl}^-$$), so the interionic distance in LiF is smaller than in LiCl. This means LiF has a higher lattice energy and therefore a higher melting point: $$\text{LiF} > \text{LiCl}$$.

Comparing MgO and NaCl: MgO consists of $$\text{Mg}^{2+}$$ and $$\text{O}^{2-}$$ ions (charges $$+2$$ and $$-2$$), while NaCl consists of $$\text{Na}^+$$ and $$\text{Cl}^-$$ ions (charges $$+1$$ and $$-1$$). The product of charges in MgO ($$2 \times 2 = 4$$) is much greater than in NaCl ($$1 \times 1 = 1$$). Additionally, the ionic radii of $$\text{Mg}^{2+}$$ and $$\text{O}^{2-}$$ are smaller than those of $$\text{Na}^+$$ and $$\text{Cl}^-$$. Therefore, MgO has a significantly higher lattice energy and melting point: $$\text{MgO} > \text{NaCl}$$.

The correct set with both pairs in the correct order is: LiF $$>$$ LiCl and MgO $$>$$ NaCl.

Therefore, the correct answer is Option (4).