Pepsin is a proteolytic enzyme produced in the stomach that functions optimally in a highly acidic environment, typically at a pH of 1.5 to 2.5. It is secreted by gastric chief cells in an inactive form called pepsinogen. The presence of hydrochloric acid (HCl) in the stomach provides the low pH necessary to convert pepsinogen into its active form, pepsin.

A. alkaline medium: Pepsin is inactivated and eventually denatured in alkaline environments, such as those found in the small intestine where bicarbonate neutralizes stomach acid.

B. alkaline and acidic medium: Pepsin cannot function in both; it is specialized strictly for the acidic conditions of the stomach and loses all enzymatic activity in alkaline settings.

D. neutral medium: At a neutral pH (around 7.0), pepsin is inactive and begins to undergo irreversible denaturation.