**Core Concept**
The pancreatic secretion rich in bicarbonate is crucial for neutralizing gastric acid in the duodenum. This process involves the coordinated action of various hormones and neural pathways. The hormone responsible for stimulating this bicarbonate-rich secretion is secretin, a peptide hormone produced by the S cells of the duodenum.

**Why the Correct Answer is Right**
Secretin acts on the pancreas by binding to secretin receptors, specifically S1 and S2 receptors, which are G protein-coupled receptors. Upon activation, these receptors stimulate the adenylate cyclase pathway, increasing intracellular cyclic AMP (cAMP) levels. Elevated cAMP levels activate protein kinase A (PKA), which in turn phosphorylates and activates the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels. The increased chloride secretion into the pancreatic duct lumen drives water and bicarbonate secretion into the lumen, resulting in a bicarbonate-rich pancreatic juice. This mechanism helps to neutralize the acidity of the gastric contents entering the duodenum.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrin stimulates gastric acid secretion, not bicarbonate-rich pancreatic secretion.
**Option B:** Cholecystokinin (CCK) primarily stimulates pancreatic enzyme secretion, not bicarbonate-rich secretion.
**Option C:** Vasoactive intestinal peptide (VIP) has a broader range of effects, including relaxation of smooth muscle and vasodilation, but is not primarily responsible for stimulating bicarbonate-rich pancreatic secretion.

**Clinical Pearl / High-Yield Fact**
Secretin's role in stimulating bicarbonate-rich pancreatic secretion highlights the importance of maintaining a neutral pH in the duodenum to facilitate nutrient absorption and prevent mucosal damage.

**Correct Answer:** D. Secretin.