Final answer:
Pepsin's activity is the highest at its optimum pH of 1.5, which is significantly more acidic than the neutral pH of 7.4 where many other enzymes are most active.
Step-by-step explanation:
Pepsin is a digestive enzyme with an optimum pH of 1.5, which means this is the pH at which its activity is the highest. While the pH in the stomach, where pepsin functions, is very acidic, this is ideal for pepsin to catalyze the digestion of proteins into smaller molecules effectively. If the pH deviates significantly from this optimum, pepsin's activity dramatically decreases. In environments with a higher pH, such as pH 2 or above, pepsin is not as active as it is at its optimum pH.