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

All the alkali metal ions exist as hydrated ions M+(H₂O)ₙ in their aqueous solutions. The degree of hydration decreases as the ionic size increases when we go down the group. Since the mobility of ions is inversely proportional to the size of their hydrated ions, the increasing order of ionic mobility is Li+ < Na+ < K+ < Rb+. Therefore, Li+ has the lowest ionic mobility.