Final answer:
Option C is correct; only the pH of the normal solution will decrease when diluted with water, as the buffered solution is designed to maintain a stable pH.
Step-by-step explanation:
When considering a buffered and a normal solution, each with a pH of 10, that are both diluted with water, the buffered solution is designed to maintain a relatively constant pH when diluted, whereas the pH of the normal solution is likely to decrease. This is because the buffer solution contains a weak acid and its conjugate base (or a weak base and its conjugate acid), which work together to accommodate changes in hydrogen ion concentration, thereby stabilizing the pH even when the solution is diluted or when small amounts of acid or base are added. As per the Henderson-Hasselbalch equation (pH = pKa + log([base]/[acid])), altering the concentration of the buffer components through dilution still maintains the pH within a relatively narrow range, provided the buffer's capacity is not exceeded. On the other hand, the pH of a normal solution lacking these buffering components will change more significantly upon dilution. Thus, the pH of the buffered solution will likely remain unchanged, but the pH of the normal solution will decrease. Therefore, the best answer to the student's question is option C: only the pH of the normal solution will decrease.