Ionic mobility of which of the following alkali metal ions is lowest when aqueous solution of their salts are put under an electric field ?

Solution :
Key concept More the extent of hydration, lesser is the ionic mobility.
In all the alkali metals, $$L{i^ + }\,ion$$   is smallest. Thus, extent of hydration is maximum in $$L{i^ + }\,ion.$$
i.e. the dissolution of $$L{i^ + }$$  in water occurs and get hydrated. Smaller the size of a cation, greater is the extent of hydration and lesser is the ionic mobility.