A mixture of weak acid and its salt is

Explanation

An acidic buffer is a solution that consists of a weak acid and its salt, which helps to maintain a stable pH level when small amounts of acid or base are added. This is the correct answer because the mixture described in the question specifically forms an acidic buffer. An alkaline buffer, on the other hand, consists of a weak base and its salt, not a weak acid. A neutral buffer is not typically created from a weak acid and its salt, as these combinations generally result in an acidic buffer. Therefore, option D 'All of the above' is incorrect because only the acidic buffer option accurately describes the given mixture.