Which of the following is not a buffer ?
`HCN +NaCN, HCl+ NaCl ,NH_4OH+ NH_4Cl`

Explanation

To determine which of the following options is not a buffer solution, we need to understand what a buffer is. A buffer solution is typically composed of either a weak acid and its salt with a strong base or a weak base and its salt with a strong acid.

Let's analyze each option step by step:

### Step 1: Analyze HCN + NaCN
- **HCN (Hydrogen Cyanide)**: This is a weak acid.
- **NaCN (Sodium Cyanide)**: This is the salt of HCN with a strong base (NaOH).
- **Conclusion**: Since this is a combination of a weak acid and its salt, HCN + NaCN forms a buffer solution.
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