The ease of adsorption of the hydrated alkali metal ions on an ion exchange resins fallows the order:

Given options are

$$(A)Rb^{+}< K^{+}< Na^{+}< Li^{+}\\(B)K^{+}< Na^{+}< Rb^{+}< Li\\(C)Na^{+}< Li^{+}< K^{+}< Rb^{+}\\(D)Li^{+}< K^{+}< Na^{+}< Rb^{+}$$

Ease of adsorption of the hydrated alkali metal ions on an ion exchange resin decreases as the size of alkali metal ion increases

We know,

The order of the size of alkali metal ion follows the order

$$Li^{+}< K^{+}< Na^{+}< Rb^{+}$$

Therefore, the ease of adsorption follows the order

$$Rb^{+}< K^{+}< Na^{+}< Li^{+}$$

Hence, the correct option is (A)