The correct order of hydration enthalpies of alkali metal ions is:

Options

A. `Na^(o+)`
B. `Li^(o+)`
C. `Cs^(o+)`
D. `K^(o+)`

Explanation

To determine the correct order of hydration enthalpies of alkali metal ions, we need to consider the factors that influence hydration enthalpy, particularly the size of the ions.

### Step-by-Step Solution:

1. **Understand Hydration Enthalpy**: Hydration enthalpy refers to the energy released when an ion is surrounded by water molecules. It is influenced by the charge of the ion and its size.

2. **Identify Alkali Metal Ions**: The alkali metal ions we are considering are Li⁺ (Lithium), Na⁺ (Sodium), K⁺ (Potassium), Rb⁺ (Rubidium), and Cs⁺ (Cesium).

3. **Consider the Charge**: All alkali metal ions have a +1 charge. Since the charge is the same for all these ions, we will focus on their sizes.

4. **Analyze Ionic Sizes**: As we move down the group in the periodic table from Li to Cs, the size of the ions increases:
- Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

5. **Relate Size to Hydration Enthalpy**: Hydration enthalpy is inversely proportional to the size of the ion. This means that smaller ions will have higher hydration enthalpy because they can interact more strongly with water molecules.

6. **Establish the Order**: Based on the size of the ions, the order of hydration enthalpy from highest to lowest is:
- Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

### Final Answer:
The correct order of hydration enthalpies of alkali metal ions is:
**Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺**