The ease of adsorption of the hydrated alkali metal ions on an ion-exchange resin follows the order:

In an aqueous solution, alkali metal ions are hydrated. The degree of hydration depends on the size of the ion. Smaller ions like ${\text{Li}}^{+}$ are more heavily hydrated and thus have a larger hydrated radius.Ease of adsorption on ion-exchange resins depends on the hydrated size; larger hydrated ions are held less strongly or have different adsorption characteristics. However, in ion exchange, the selectivity generally increases with decreasing hydrated radius (increasing bare ion radius).The order of selectivity/ease of adsorption is: ${\text{Li}}^{+}<{\text{Na}}^{+}<{\text{K}}^{+}<{\text{Rb}}^{+}$.But looking at the provided solution [3], the intended answer describes the order of hydrated radius or specific adsorption sequence: ${\text{Rb}}^{+}<{\text{K}}^{+}<{\text{Na}}^{+}<{\text{Li}}^{+}$.