Respuesta :
D is the correct answer. Sodium hydroxide, a base, is added to the solution, but the pH of the solution does not change. When we talk about a buffer solution it is about resistance in change of pH when small quantity of alkali or acid added to it. Sodium hydroxide is a weak base so adding it to pH less than 7 is more likely to resist.
Alternative Answer is: Acetic acid is added to the solution, but the pH of the solution does not change.