**Core Concept**
Secretin is a hormone produced by the S cells of the duodenum in response to acidic chyme entering the small intestine. It plays a crucial role in pancreatic and biliary secretions, promoting the release of bicarbonate-rich fluid to neutralize the acid and maintain a pH suitable for enzyme activity.

**Why the Correct Answer is Right**
Secretin acts on the pancreas to stimulate the release of bicarbonate-rich fluid, which helps to neutralize the acidic chyme entering the small intestine. This action is essential for creating an optimal environment for the action of pancreatic enzymes. Secretin also inhibits gastric emptying, allowing for a more controlled release of acidic chyme into the small intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because secretin does not stimulate the release of gastrin, which is a hormone that increases gastric acid secretion.
**Option B:** This statement is incorrect because secretin acts on the pancreas to stimulate the release of bicarbonate-rich fluid, not to stimulate the release of digestive enzymes.
**Option C:** This statement is incorrect because secretin is not a gastrointestinal hormone that stimulates the release of insulin.

**Clinical Pearl / High-Yield Fact**
Secretin is often used clinically to stimulate pancreatic function and diagnose pancreatic insufficiency. It is also used in research to study the regulation of pancreatic secretions.

**Correct Answer: D.**