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+
The correct option is D.