The aqueous solution of which one of the following compounds maintain its pH constant?

Explanation

A buffer solution is designed to maintain a stable pH when small amounts of an acid or base are added. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. In this question, the correct answer is NH4OH and NH4Cl. NH4OH is a weak base, and NH4Cl provides the conjugate acid (NH4+), forming a buffer system that resists changes in pH. The other options do not meet the criteria for forming effective buffer solutions: CH3COOH and (NH4)2SO4 do not form a compatible acid-base pair; NH4NO3 and KNO3 are salts that do not buffer; and NH4OH and NaCl lack the necessary acidic component.