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, the smallest ion (${\text{Li}}^{+}$) has the highest charge density and becomes the most heavily hydrated. The effective size of the hydrated ${\text{Li}}^{+}$ ion is the largest among alkali metals, causing it to move the slowest (lowest mobility) in an electric field.