Final answer:
Pepsin is most active b. in the stomach due to the acidic environment that is optimal for breaking down proteins into smaller components.
Step-by-step explanation:
Pepsin would be most active in the stomach, where the highly acidic environment (pH of about 1.5) is ideal for its function. In the mouth, the enzyme salivary amylase begins the digestion of carbohydrates, and the mouth's environment is not suitable for the activity of pepsin. Pepsin is a proteolytic enzyme, which means it is involved in the breakdown of proteins. This enzyme is secreted by the chief cells in the stomach as pepsinogen and is activated to pepsin in the presence of hydrochloric acid (HCl), which is secreted by parietal cells. The highly acidic pH of the stomach enables pepsin to catalyze the cleavage of peptide bonds, leading to the breakdown of proteins into smaller polypeptides and amino acids.