In the laboratory, `H_(2)O_(2)` is prepared by the action of

Options

A. `MnO_(2)` is added to dilute cold `H_(2)SO_(4)`
B. `BaO_(2)` is added to `CO_(2)` bubbling through cold water
C. `PbO_(2)` is added to an acidified solution of `KMnO_(4)`
D. `Na_(2)O_(2)` is added to boiling water

Answer

To solve the question regarding the laboratory preparation of hydrogen peroxide (H₂O₂), we will analyze the options provided and identify the correct method of preparation.

### Step-by-Step Solution:

1. **Understanding the Options**: We have four options to consider for the preparation of H₂O₂ in the laboratory:
- Option A: MnO₂ is added to dilute cold H₂SO₄.
- Option B: BaO₂ is added to CO₂ bubbling through cold water.
- Option C: PbO₂ is added to an acidified solution of KMnO₄.
- Option D: NaO₂ is added to boiling water.

2. **Analyzing Option A**:
- MnO₂ (manganese dioxide) does not react with dilute cold H₂SO₄ to produce H₂O₂. This option is incorrect.

3. **Analyzing Option B**:
- Barium peroxide (BaO₂) can react with carbon dioxide (CO₂) when bubbled through cold water. The reaction produces barium carbonate (BaCO₃) and hydrogen peroxide (H₂O₂). This method is valid for the preparation of H₂O₂. Therefore, this option is correct.

4. **Analyzing Option C**:
- Lead dioxide (PbO₂) does not react with an acidified solution of potassium permanganate (KMnO₄) to produce H₂O₂. This option is incorrect.

5. **Analyzing Option D**:
- Sodium peroxide (NaO₂) does not produce H₂O₂ when added to boiling water. This option is incorrect.

6. **Conclusion**:
- The only correct method for the laboratory preparation of hydrogen peroxide from the given options is **Option B**: BaO₂ is added to CO₂ bubbling through cold water.

### Final Answer:
**BaO₂ is added to CO₂ bubbling through cold water.**