**Core Concept**
Secretin is a hormone released by the duodenum in response to the presence of acidic chyme from the stomach. It plays a crucial role in regulating pancreatic and bile duct secretions, promoting the release of bicarbonate-rich fluid to neutralize the acidic environment.

**Why the Correct Answer is Right**
Secretin is produced by S cells, which are specialized enteroendocrine cells located in the duodenum and jejunum. These cells contain secretin granules that release the hormone in response to the acidity of the chyme. Secretin then binds to receptors in the pancreas and bile ducts, triggering the release of bicarbonate-rich secretions that neutralize the acid and create a favorable environment for digestion.

**Why Each Wrong Option is Incorrect**
**Option A:** I cells, on the other hand, are responsible for producing cholecystokinin (CCK), a hormone that stimulates the release of digestive enzymes and bile from the pancreas and gallbladder.

**Option B:** T cells are not directly related to the production of secretin. They are a type of immune cell involved in cell-mediated immunity.

**Option C:** M cells are specialized epithelial cells that play a role in antigen uptake and presentation in the gut-associated lymphoid tissue (GALT). They are not involved in the production of secretin.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the release of secretin is triggered by the acidity of the chyme, which is why antacids and acid-reducing medications can affect the release of this hormone.

**Correct Answer:**
✓ Correct Answer: D. S cells.