Which hormone is responsible for the secretion of bicarbonate-rich juice from the pancreas?

The correct choice is secretin, as it is a hormone that plays a crucial role in the digestive process, specifically in regulating the secretion of bicarbonate-rich fluid from the pancreas. When acidic chyme from the stomach enters the small intestine, secretin is released into the bloodstream. This hormone stimulates the pancreas to release a bicarbonate-rich pancreatic juice, which serves to neutralize the acidity of the chyme. This neutralization is essential for optimal enzyme activity during digestion, as the enzymes that function in the small intestine work best at a more neutral pH.

In contrast, insulin is primarily involved in regulating glucose levels in the blood, and its role is not directly related to the secretion of pancreatic digestive juices. Gastrin is a hormone that stimulates gastric acid secretion in the stomach and does not have a direct effect on the pancreas. Cholecystokinin, while it does stimulate the release of digestive enzymes from the pancreas, is not the main hormone responsible for the secretion of bicarbonate; that role uniquely belongs to secretin.