Trypsinogen is converted to active trypsin by the enzyme enterokinase which is secreted by the intestinal mucosa. C is the correct answer.

Introduction:
The release of pancreatic juice is regulated by various hormones and signals that coordinate the digestive process. These signals help to stimulate the pancreas to produce and release pancreatic juice, which contains digestive enzymes that aid in the digestion of carbohydrates, proteins, and fats.

Gastrin:
Gastrin is a hormone that is released by the stomach in response to the presence of food. It stimulates the release of pancreatic juice by acting on the pancreas directly. Gastrin triggers the production and secretion of pancreatic enzymes, including amylase, lipase, and proteases.

Secretin:
Secretin is another hormone involved in the stimulation of pancreatic juice release. It is released by the duodenum, the first part of the small intestine, in response to the acidic chyme (partially digested food) entering from the stomach. Secretin stimulates the pancreas to release bicarbonate ions, which help to neutralize the acidic chyme. This secretion of bicarbonate-rich pancreatic juice aids in the digestion process.

Trypsinogen:
Trypsinogen is not directly involved in the stimulation of pancreatic juice release. It is an inactive precursor enzyme of trypsin, which is one of the pancreatic proteases. Trypsinogen is released by the pancreas into the small intestine, where it is activated by an enzyme called enterokinase. Once activated, trypsin can further activate other pancreatic enzymes, including chymotrypsinogen and procarboxypeptidase, to facilitate protein digestion.

Cholecystokinin:
Cholecystokinin (CCK) is a hormone released by the duodenum in response to the presence of fat and protein in the small intestine. CCK stimulates the release of pancreatic enzymes, including lipase and proteases, to aid in the digestion of fats and proteins. It also promotes the contraction of the gallbladder for the release of bile, which is important for fat digestion.

Conclusion:
Out of the given options, trypsinogen is not involved in the stimulation of pancreatic juice release. While trypsinogen is a precursor enzyme that is released by the pancreas, it requires activation by enterokinase in the small intestine to become trypsin, which can then activate other pancreatic enzymes. Gastrin, secretin, and cholecystokinin play important roles in stimulating the release of pancreatic juice and coordinating the digestive process.