An aqueous solution of nitrous acid `(HNO_(2))` , free of salts , can be obtained from the reaction

Options

A. `Ba(NO_(2))_(2) +H_(2)SO_(4) to `
B. `NaNO_(2) + H_(2)SO_(4) overset("Cold ") to `
C. `NH_(4)NO_(2) + H_(2)SO_(4) to `
D. `KNO_(3) +HNO_(3) to `

Explanation

To determine which reaction yields an aqueous solution of nitrous acid (HNO₂) free of salts, we can analyze the provided options step by step.

### Step-by-Step Solution:

1. **Identify the Reactions**: We need to evaluate the given reactions to see which one produces nitrous acid (HNO₂) without any accompanying salts.

2. **Evaluate Option 1**:
- Reaction: \( \text{Ba(NO}_2\text{)}_2 + \text{H}_2\text{O} \)
- This reaction produces \( 2 \text{HNO}_2 + \text{BaSO}_4 \).
- **Result**: This reaction produces nitrous acid along with a salt (BaSO₄).

3. **Evaluate Option 2**:
- Reaction: \( \text{NaNO}_2 + \text{H}_2\text{SO}_4 \)
- This reaction yields \( \text{Na}_2\text{SO}_4 + \text{H}_2\text{O} \).
- **Result**: No nitrous acid is produced here.

4. **Evaluate Option 3**:
- Reaction: \( \text{NH}_4 + \text{NaNO}_2 \)
- This reaction produces \( \text{NO}_2 + \text{(NH}_4)_2\text{SO}_3 + \text{H}_2\text{O} \).
- **Result**: No nitrous acid is produced here.

5. **Evaluate Option 4**:
- Reaction: \( \text{KNO}_3 + \text{HNO}_3 \)
- This reaction yields a complex of \( \text{HNO}_3 \).
- **Result**: No nitrous acid is produced here.

6. **Conclusion**:
- After evaluating all options, only Option 1 produces nitrous acid (HNO₂) but also produces a salt (BaSO₄). However, it is the only option that yields nitrous acid. Therefore, if we strictly need an aqueous solution of nitrous acid free of salts, we find that none of the options perfectly meet the criteria. However, Option 1 is the closest as it does produce nitrous acid.

### Final Answer:
The reaction that yields nitrous acid (HNO₂) is:
- **Option 1**: \( \text{Ba(NO}_2\text{)}_2 + \text{H}_2\text{O} \rightarrow 2 \text{HNO}_2 + \text{BaSO}_4 \)