Which from the following is NOT a buffer solution?

A buffer solution is formed by mixing a weak acid and its conjugate base (salt with a strong base) or a weak base and its conjugate acid (salt with a strong acid).

$\text{CH}_3\text{COOH} + \text{CH}_3\text{COONa}$ is an acidic buffer because it contains a weak acid ($\text{CH}_3\text{COOH}$) and its salt with a strong base ($\text{CH}_3\text{COONa}$).

$\text{NH}_4\text{OH} + \text{NH}_4\text{Cl}$ is a basic buffer because it contains a weak base ($\text{NH}_4\text{OH}$) and its salt with a strong acid ($\text{NH}_4\text{Cl}$).

$\text{HCOOH} + \text{HCOONa}$ is an acidic buffer because it contains a weak acid ($\text{HCOOH}$) and its salt with a strong base ($\text{HCOONa}$).

$\text{CH}_3\text{COOH} + \text{NaCl}$ is a mixture of a weak acid and a salt of a strong acid and strong base. It does not contain the conjugate base of the weak acid ($\text{CH}_3\text{COO}^{-}$), so it cannot act as a buffer solution.

Answer: $\text{CH}_3\text{COOH} + \text{NaCl}$ in water.