In an aqueous solution, the degree of hydration of an ion depends on its charge density (charge-to-size ratio). The sizes of the bare alkali metal ions increase down the group: $\text{Na}^+ < \text{K}^+ < \text{Rb}^+ < \text{Cs}^+$. As a result, the smaller $\text{Na}^+$ ion is the most extensively hydrated, forming the largest hydrated ion, whereas the larger $\text{Cs}^+$ ion is the least hydrated, forming the smallest hydrated ion. Since ionic mobility is inversely proportional to the effective size of the hydrated ion, the smallest hydrated ion ($\text{Cs}^+$) will have the highest mobility, and the largest hydrated ion ($\text{Na}^+$) will have the lowest mobility. Therefore, the correct order of ionic mobility in aqueous solution is $\text{Cs}^+ > \text{Rb}^+ > \text{K}^+ > \text{Na}^+$.