Which solution is most resistant to pH changes when an acid or base is added?

The solution that is most resistant to pH changes when an acid or base is added is comprised of a weak acid and its conjugate base, which is exactly what the selected choice represents. In this case, the weak acid is acetic acid (HC2H3O2) and the conjugate base is sodium acetate (NaC2H3O2).

This combination functions as a buffer solution. Buffer solutions work by containing both a weak acid and its conjugate base in similar concentrations. When an acid is added to the solution, the conjugate base reacts with the added hydrogen ions (H+) to minimize the change in pH. Conversely, if a base is added, the weak acid can donate protons to counteract the base and again stabilize the pH level. This dual capability allows the buffer solution to effectively resist changes in pH.

In contrast, the other combinations listed do not exhibit the same buffering capacity. A strong acid with its corresponding salt (as seen in the first choice) will not effectively neutralize additional acids or bases. Similarly, adding a strong base to an acidic solution (as in the second choice) or having just a simple salt or a strong base (as with sodium carbonate in the fourth choice)