In a mammal, the release of pancreatic juice from the pancreas is triggered by

The correct option is secretin.

In mammals, the regulation of digestion involves both neural and hormonal control systems that coordinate the release of digestive enzymes and juices precisely when food is present.

When acidic chyme (partially digested food from the stomach) enters the duodenum (the first part of the small intestine), it stimulates specialized S-cells in the duodenal mucosa to secrete the hormone secretin into the bloodstream.
Secretin travels through the circulatory system to the pancreas, where it binds to specific receptors on pancreatic duct cells.
This binding triggers the pancreatic duct cells to release a watery, bicarbonate-rich pancreatic juice. The high concentration of bicarbonate ions (${\mathrm{HCO}}_3^{-}$) helps neutralize the highly acidic gastric juices entering the duodenum from the stomach, creating an optimal, slightly alkaline pH environment for pancreatic digestive enzymes to function.

Let us briefly look at the other options to understand why they are not correct:
Cholecystokinin (CCK): Although it is also a duodenal hormone that acts on the pancreas, CCK primarily stimulates the acinar cells of the pancreas to release enzyme-rich pancreatic secretions (like lipases, amylases, and proteases) rather than the overall alkaline fluid component of pancreatic juice, and it stimulates gallbladder contraction.
Trypsinogen: This is an inactive enzyme precursor (zymogen) secreted in pancreatic juice, not a regulatory hormone or trigger.
Enterokinase (enteropeptidase): This is an enzyme anchored in the duodenal mucosa that activates trypsinogen into active trypsin; it does not trigger the secretion of pancreatic juice.