The solubility of ionic solids decreases when hydration energy of ions increases.
The solubility of metal halides depends on their nature, lattice enthalpy and hydration enthalpy of the individual ions. Amongst flurides of alkali metals, the lowest solubility of LiF in water is due to (a) Ionic nature fo lithium fluoride (b) high lattice enthalpy (c) high hydration enthalpy for lithium ion (d) low ionisation enthalpy of lithium atom