Secretion of pancreatic juice is triggered by

The correct option is secretin.

Step-by-Step Explanation:

1. Understanding the Role of the Pancreas:
The pancreas is a vital digestive gland that secretes pancreatic juice containing digestive enzymes and bicarbonate ions into the duodenum (the first part of the small intestine). This secretion is highly regulated by both nervous and hormonal signals to ensure it coincides with the arrival of food.

2. Hormonal Regulation of Pancreatic Secretion:
When acidic chyme (partially digested food from the stomach) enters the duodenum, it stimulates specialized endocrine cells in the duodenal mucosa to release the hormone secretin into the bloodstream.

3. Mechanism of Secretin:
Once released, secretin travels through the blood to the pancreas. It binds to receptors on pancreatic duct cells, triggering them to secrete a watery fluid rich in bicarbonate ions (${\mathrm{HCO}}_3^{-}$). Bicarbonate is crucial because it neutralizes the highly acidic gastric juice entering from the stomach, creating an optimal alkaline pH for pancreatic digestive enzymes to function.

4. Evaluating Other Options:
- Enterogastrone: A hormone secreted by the duodenal mucosa that inhibits gastric secretion and motility, rather than stimulating pancreatic secretion.
- Gastrin: A hormone secreted by the stomach (G-cells) that stimulates the secretion of gastric acid (HCl) by parietal cells.
- Enterokinase (Enteropeptidase): An enzyme in the brush border of the duodenal mucosa that activates trypsinogen into active trypsin, which is a local enzymatic process rather than a trigger for the secretion of pancreatic juice itself.

Therefore, the secretion of pancreatic juice is triggered primarily by the hormone secretin.