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

In aqueous solution, smaller ions have a higher charge density and thus become more heavily hydrated.${\text{Li}}^{+}$ is the smallest and most hydrated, resulting in a large effective radius and low mobility.${\text{Rb}}^{+}$ is larger, less hydrated, and has a smaller effective radius in water.Therefore, mobility increases down the group: ${\text{Li}}^{+}<{\text{Na}}^{+}<{\text{K}}^{+}<{\text{Rb}}^{+}$.