The oxide, which is best soluble in `H_(2)O` is

Options

A. `Ba(OH)_(2)`
B. `Mg(OH)_(2)`
C. `Sr(OH)_(2)`
D. `Ca(OH)_(2)`

Explanation

To determine which oxide is best soluble in water, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Oxides**: First, we need to identify the oxides of the alkaline earth metals (group 2 elements) that we are considering. Common oxides include:
- Magnesium oxide (MgO)
- Calcium oxide (CaO)
- Strontium oxide (SrO)
- Barium oxide (BaO)

2. **Understand Solubility Trends**: The solubility of metal hydroxides in water generally increases down the group in the alkaline earth metals. This is due to the increasing ionic size, which allows for better hydration of the larger cations.

3. **Consider Hydroxides**: While the question asks about oxides, it is important to note that the hydroxides of these metals are often more soluble than their corresponding oxides. For example:
- Barium hydroxide (Ba(OH)₂) is highly soluble in water.
- Strontium hydroxide (Sr(OH)₂) is also quite soluble.
- Calcium hydroxide (Ca(OH)₂) has moderate solubility.
- Magnesium hydroxide (Mg(OH)₂) is only slightly soluble.

4. **Compare Oxides**: Among the oxides:
- MgO is sparingly soluble in water.
- CaO reacts with water to form Ca(OH)₂, which is more soluble.
- SrO also reacts with water to form Sr(OH)₂.
- BaO reacts with water to form Ba(OH)₂, which is the most soluble of the hydroxides.

5. **Conclusion**: Since the question asks for the oxide that is best soluble in water, we conclude that barium oxide (BaO) is the best option because it forms barium hydroxide (Ba(OH)₂), which is highly soluble in water.

### Final Answer:
The oxide which is best soluble in water is **Barium oxide (BaO)**.