The solubility of metal halides depends on their nature, lattice enthalpy and
hydration enthalpy of the individual ions. Amongst fluorides of alkali metals,
the lowest solubility of LiF in water is due to
(i) Ionic nature of lithium fluoride
(ii) High lattice enthalpy
(iii) High hydration enthalpy for lithium-ion.
(iv) Low ionisation enthalpy of the lithium atom
Option (ii) High lattice enthalpy is the answer.