Compare melting points of $\mathrm{LiF}$ and $\mathrm{LiCl}$

For ionic compounds, melting point generally increases with stronger ionic attraction.

• Both salts contain $\mathrm{Li^+}$.
• Compare anions: $\mathrm{F^-}$ is much smaller than $\mathrm{Cl^-}$.
• Smaller ions give higher lattice energy because interionic distance is smaller.

Hence, $\mathrm{LiF} > \mathrm{LiCl} \quad \text{(in melting point)}$

Also, $\mathrm{LiCl}$ has more covalent character than $\mathrm{LiF}$ (due to larger, more polarizable $\mathrm{Cl^-}$), which further lowers its melting point.

Compare melting points of $\mathrm{MgO}$ and $\mathrm{NaCl}$

Melting point depends strongly on lattice energy.

• $\mathrm{MgO}$ consists of $\mathrm{Mg^{2+}}$ and $\mathrm{O^{2-}}$.
• $\mathrm{NaCl}$ consists of $\mathrm{Na^+}$ and $\mathrm{Cl^-}$.

Since lattice energy is roughly proportional to $\frac{|z_+ z_-|}{r}$ where $z_+$ and $z_-$ are ionic charges, $\mathrm{MgO}$ has much stronger electrostatic attraction because:

• charges are $+2$ and $-2$ in $\mathrm{MgO}$,
• charges are only $+1$ and $-1$ in $\mathrm{NaCl}$.

Therefore, $\mathrm{MgO} > \mathrm{NaCl} \quad \text{(in melting point)}$

Match with options

We found: $\mathrm{LiF} > \mathrm{LiCl}$ and $\mathrm{MgO} > \mathrm{NaCl}$

This corresponds to Option D.

Final Answer

$\boxed{\text{D}}$