Answer:
D. Stomach
Explanation:
One of the most important digestive enzymes that aids digestion by breaking down of ingested food, most especially proteins is pepsin, which is usually referred to as "stomach enzyme".
Pepsin is found secreted in the stomach.
The pH of the stomach is usually low, between 1.5 to 2 (acidic). This acidic environment of the stomach actually favors this stomach enzyme, as it activated the pepsin to perform it's function.
Based on the observations made by the students, it can be concluded that pepsin aids digestion most in the stomach. It's pH requirement to function optimally matches with the pH of the environment of the stomach.