Final answer:
The correct set from the given options, in which both pairs are in the correct order of melting point, is option d. LiF > LiCl; MgO > NaCl. Therefore, the correct option is D.
Step-by-step explanation:
The correct set from the following in which both pairs are in the correct order of melting point is:
d. LiF > LiCl; MgO > NaCl
The melting points of compounds are influenced by the strength of the intermolecular forces between their constituent particles. In this case, LiF has a higher melting point than LiCl because LiF has stronger ionic bonds due to the higher electronegativity difference between Li and F compared to Li and Cl. Similarly, MgO has a higher melting point than NaCl because MgO has stronger ionic bonds due to higher charge density of Mg2+ compared to Na+.