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

In aqueous solution, ions undergo hydration. Smaller ions like ${\text{Li}}^{+}$ have the highest charge density and thus attract more water molecules, resulting in a very large hydrated radius. A larger hydrated ion moves more slowly in an electric field. Therefore, ${\text{Li}}^{+}$ has the lowest ionic mobility among the alkali metal ions in water.