**Core Concept**
Secretin is a hormone released by the duodenal mucosa in response to the entry of acidic chyme from the stomach into the small intestine. It plays a crucial role in the regulation of pancreatic and bile duct secretions, facilitating the digestion and absorption of nutrients.

**Why the Correct Answer is Right**
Secretin acts on the pancreas and bile duct to stimulate the release of bicarbonate-rich secretions, which help neutralize the acidic chyme and create an optimal environment for enzyme activity. This is mediated through the activation of the secretin receptor, a G-protein coupled receptor that triggers an increase in intracellular cyclic AMP (cAMP) levels. The subsequent increase in cAMP activates protein kinase A (PKA), which phosphorylates and activates the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, leading to the secretion of bicarbonate ions.

**Why Each Wrong Option is Incorrect**

**Option A:** This option might be incorrect if it suggests that secretin is primarily involved in the regulation of pancreatic enzyme secretion, rather than bicarbonate secretion.
**Option B:** This option might be incorrect if it implies that secretin acts solely on the pancreas, without considering its effects on the bile duct.

**Clinical Pearl / High-Yield Fact**
Secretin is often used as a diagnostic tool in the evaluation of pancreatic function, particularly in the diagnosis of pancreatic insufficiency. It can also be used to stimulate the release of bicarbonate-rich secretions in patients with duodenal ulcers or other gastrointestinal disorders.

**Correct Answer:** C. Secretin stimulates the release of bicarbonate-rich secretions from the pancreas and bile duct.