Which of the following constitutes a buffer ?

Options

A. `HNO_(2)` and `NaNO_(2)`
B. NaOH and NaCl
C. `HNO_(3)` and `NH_(4)NO_(3)`
D. HCl and KCl

Explanation

To determine which of the given options constitutes a buffer solution, we need to understand the criteria for a buffer solution. A buffer solution is typically formed by a weak acid and its salt or a weak base and its salt. Buffer solutions resist changes in pH upon the addition of small amounts of acids or bases due to the presence of equilibrium between the weak acid/base and its conjugate salt.

### Step-by-Step Solution:

1. **Identify the Options**: We need to analyze the given compounds to see if they fit the criteria for a buffer solution.

2. **Evaluate HCl**:
- HCl is a strong acid.
- Since buffers require a weak acid or a weak base, HCl cannot constitute a buffer.

3. **Evaluate HNO3**:
- HNO3 is also a strong acid.
- Similar to HCl, it does not meet the criteria for a buffer solution.

4. **Evaluate NaOH**:
- NaOH is a strong base.
- Buffers require a weak base or a weak acid, so NaOH cannot constitute a buffer.

5. **Evaluate HNO2**:
- HNO2 is a weak acid.
- When HNO2 reacts with a strong base like NaOH, it forms the salt NaNO2 and water.
- The combination of HNO2 (weak acid) and NaNO2 (its salt) meets the criteria for a buffer solution.

6. **Conclusion**:
- Among the options provided, only HNO2 along with its salt (NaNO2) constitutes a buffer solution.

### Final Answer:
- The buffer solution is constituted by HNO2 and its salt NaNO2.