(A) : Lithium salts are most hydrated amongst alkali metal salts.
(R) : The hydration enthalpy of alkali metal ions increase with decrease in ionic size.

Lithium is the smallest in size among the alkali metals. Hence, Li⁺  ion can polarize water molecules more easily than other alkali metals. As a result, water molecules get attached to lithium salts as water of crystallization. 
Hence, lithium salts such as trihydrated lithium chloride (LiCl.3H₂0) are commonly hydrated. As the size of the ions increases, their polarizing power decreases. Thus hydration enthalpies decreases.Hence, other alkali metal ions usually form anhydrous salts.
Hence both Assertion and Reason are correct also Reason is correct explanation for Assertion.