Which of the following is not a buffer soltuion ?

Buffer solution is used to prevent the change in the pH of a solution after adding a little amount of acid or base in it. There are two types of buffers:

(I) equimolar mixture of weak acid and its salt with a strong base.
(II) equimolar mixture of weak base and its salt with a strong acid.

(CH₃COOH/CH₃COONa) and (HCOOH/HCOONa) are the buffer solutions of a weak acid and its salt with a strong base. NH₄OH/NH₄Cl is a buffer solution of a weak base and its salt with a strong acid. HCl/NaCl is not a buffer solution because HCl is strong acid and NaCl is a neutral salt.