The increasing order of ionic radii of alkali metals is

${\text{Li}}^{\text{+}}{\text{ < Na}}^{\text{+}}{\text{ < K}}^{\text{+}}{\text{ < Rb}}^{\text{+}}{\text{ < Cs}}^{\text{+}}\text{.}$

In aqueous solution which of the following moves the slowest?

Li⁺ is heavily hydrated in water because of being very small in size. Hence, hydrated, Li⁺ produced moves the slowest in aqueous solution alkali among all metal cations.