To get a better understanding of the answer,let's take use of Fajan's rule which says that no bond is neither purely covalent or purely ionic.
For ionic compounds MP and BP are higher than covalent compounds. So in order to find the one with higher MP,it's enough to find the most ionic compound among the given options.
In LiF,LiCl,LiBr and LiI,The cation is same and anion only differs.According to Fajans rule Smaller anion and larger cation favours ionic bonding and vice versa. Since Fluoride is the smallest anion,LiF is the most ionic compound and hence it is the one with highest MP from the options.Hope it helps
