Final answer:
The pancreas secretes trypsinogen, which is converted to trypsin in the duodenum by the enzyme enterokinase. This enzyme initiates a chain reaction, activating other protein-digesting enzymes necessary for nutrient absorption.
Step-by-step explanation:
Trypsinogen is an enzyme that the pancreas secretes into the small intestine, and it is then converted to trypsin. The enzyme responsible for this conversion is not hydrochloric acid, pepsin, or protease but rather enterokinase, which is secreted by the intestinal mucosa. So, the correct answer is D. Enterokinase.
Pancreatic juice contains several inactive enzymes, such as trypsinogen and chymotrypsinogen, which are activated through a chain of events once they reach the small intestine. Here, the intestinal brush border enzyme enterokinase stimulates the activation of trypsin from trypsinogen. Once trypsin is formed, it can then activate other pancreatic enzymes, including more trypsinogen and chymotrypsinogen, into their active forms, trypsin and chymotrypsin, respectively, ultimately aiding in protein digestion.