The sequence of ionic mobility in the aqueous solution is

Smaller the size of an ion, more strongly it is hydrated and less is the ionic mobility. The sizes of the unhydrated ions increase in the order: ${\mathrm{Na}}^{+}<K^{+}<R{b}^{+}<C{s}^{+}$

The sizes of the hydrated ions will be in the order: ${\mathrm{Cs}}^{+}<R{b}^{+}<K^{+}<N{a}^{+}$

Hence, their ionic mobilities will be in the order: ${\mathrm{Cs}}^{+}>R{b}^{+}>K^{+}>N{a}^{+}$