Final answer:
Pepsin will have the highest activity at its optimum pH of around 1.5, which is tailored to the acidic conditions of the stomach.
Step-by-step explanation:
Pepsin is a digestive enzyme that exhibits maximum activity in acidic conditions. The optimum pH for pepsin, which is the pH at which its activity is highest, is around 1.5. This is significantly lower than the biological pH of 7.4, where most enzymes show peak activity. Since pepsin functions in the stomach, it's adapted to work best in the highly acidic environment here. If the pH deviates from this optimum, the enzyme's activity would decrease, leading to less efficient protein digestion.