charge on ion (greater the charge higher is the lattice energy)

size of ion (smaller the size of ion greater is the lattice energy)

Among \(KCl\) and \(MgO , K\) has charge \(+1\) and \(Mg\) has charge \(+2\). Also, \(K ^{+}\) has larger ionic radii than \(Mg ^{+2}\). So, both the factors add up and the lattice energy of \(MgO\) is higher than \(KCl\).

\(LiF\) and \(LiBr\) have the same cation and same ionic charge. So lattice energy depends upon the size of anion. \(F ^{-}\) is smaller than \(Br ^{-}\), therefore, lattice energy of \(LiF\) is higher than \(LiBr\).

\(MgF _2\) has a charge \(+2\) while \(NaCl\) has a charge \(+1\). Therefore, lattice energy of \(MgF _2\) is higher than \(NaCl\)