which compound should require the highest temperature to melt? a lif b naf c licl d nacl

Answer

D. Na₂O

Lattice energy affects melting point. Higher lattice energy means higher melting point.

For ionic compounds, lattice energy is related to ionic charge and ionic radius.
In Na₂O, (O^{2 -}) has a higher charge ((z = 2)) compared to (F^{-}(z = 1)) in NaF and LiF. In LiCl, (Cl^{-}) has a larger radius than (O^{2 -}) (and lower charge).
According to Coulomb's law (U=\frac{kq_1q_2}{r}) (where (U) is lattice energy, (k) is a constant, (q_1) and (q_2) are ionic charges, (r) is the distance between ions).
For Na₂O, with higher ionic charge ((q_1 = 1) for (Na^{+}), (q_2=2) for (O^{2 -})) and relatively small ionic radii (compared to some other combinations), it has a higher lattice energy.

So Na₂O should require the highest temperature to melt.