The ionic mobility of alkali metal ions in aqueous solution is maximum for :

The alkali metal ion exist as hydrated ions M⁺(H₂O)_n in the aqueous solution. The degree of hydration, decreases with ionic size as we go down the group. Hence Li⁺ ion is mostly hydrated e.g. [Li(H₂O)₆]⁺. Since the mobility of ions is inversely proportional to the size of the their hydrated ions, hence the increasing order of ionic mobility is

Li⁺ < Na⁺ < K⁺ < Rb⁺