What is a buffer?
a. an acid-base conjugate pair that changes the reaction rate
b. an acid-base conjugate pair that changes dissociation constants. c. an acid-base conjugate pair that precipitates out salts
d. an acid-base conjugate pair that keeps the ph from changing

D. An acid-base conjugate pair that keeps the pH from changing

Explanation: the answer is D. An acid-base conjugate pair that keeps the pH from changing, because we have to keep in mind which is the scope of this solution, and the scope is maintain the pH despite the changes in the solutions or support any change when you add another solution to the buffer.

also we need to know that a Buffer achieve their resistance to the changes of the pH, because of the presence of an equilibrium between the acid HA and its conjugate base.

This equilibrium give the properties to this solution to support the changes.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.