As we move down the group, the ionic and atomic size of the metals increase. The given alkali metal ions can be arranged in the increasing order of their ionic sizes as:

Li⁺ < N⁺ < K⁺ < Rb⁺ < Cs⁺

Smaller the size of the ion, more highly it is hydrated. Since Li⁺ is the smallest in size, it becomes hydrated in the solution. On the other hand, Cs⁺ is the largest in size and so it is the least hydrated. Hence, the extent of hydration decreases in the order:

Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

Greater is the mass of the hydrated ion, lower will be its ionic mobility. Hence, Li⁺ ion has lower ionic mobility and Cs⁺ has higher ionic mobility. Thus, the given alkali metal ions can be arranged in the increasing order of their mobilities as:

Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺