The most stable compound is

The electronegativity of halogen is as : \(\mathrm{F} \gt \mathrm{Cl} \gt \mathrm{Br} \gt \mathrm{I}\) Among alkali metals the electropositive character increases as : \(\mathrm{Li} \lt \mathrm{Na} \lt \mathrm{K} \lt \mathrm{Rb} \lt \mathrm{Cs}\) Hence, LiF is the most stable compound.