**Core Concept**
The I cells of the duodenum are specialized endocrine cells that play a crucial role in regulating calcium and glucose homeostasis in the body. They secrete a hormone that is essential for the absorption of calcium in the small intestine.

**Why the Correct Answer is Right**
The correct answer is secretin. Secretin is a hormone secreted by the I cells of the duodenum in response to the arrival of acidic chyme from the stomach. It stimulates the pancreas to release bicarbonate-rich pancreatic juice, which helps to neutralize the acidity of the chyme and create an optimal environment for the digestion of proteins and fats. Secretin also inhibits gastric secretion and motility, allowing for a more controlled digestion process.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrin - This hormone is secreted by G cells in the stomach and stimulates gastric acid secretion, which is not the primary function of I cells in the duodenum.

**Option B:** Cholecystokinin (CCK) - This hormone is secreted by the duodenum in response to the presence of fat and protein in the chyme, but it is produced by I cells in the duodenum, not secretin.

**Option C:** Glucagon - This hormone is secreted by the pancreas and plays a key role in regulating blood glucose levels, but it is not secreted by I cells in the duodenum.

**Clinical Pearl / High-Yield Fact**
The duodenum is a critical region for the regulation of calcium homeostasis, and the secretion of secretin by I cells plays a key role in this process. This is why disorders that affect the duodenum or the pancreas can lead to calcium-related problems, such as osteoporosis or tetany.

**Correct Answer:** B. Cholecystokinin (CCK)