**Core Concept**
The **gastrointestinal tract** has various hormone-secreting cells that regulate digestion and nutrient absorption. **Cholecystokinin (CCK)** is one such hormone that plays a crucial role in facilitating digestion in the small intestine.

**Why the Correct Answer is Right**
**Cholecystokinin** is primarily secreted by the **duodenal mucosa**, specifically by the **I-cells** in the duodenum, in response to the presence of fatty acids and amino acids in the intestinal lumen. This hormone stimulates the release of **bile** from the gallbladder and **digestive enzymes** from the pancreas, thereby aiding in the digestion and absorption of nutrients.

**Why Each Wrong Option is Incorrect**
**Option A:** The stomach is primarily involved in the secretion of **gastric acid** and **pepsinogen**, not cholecystokinin.
**Option B:** The jejunum, while part of the small intestine, is not the primary site for CCK secretion.
**Option D:** The ileum is involved in the absorption of **bile salts** and **vitamin B12**, but it is not the main site for cholecystokinin secretion.

**Clinical Pearl / High-Yield Fact**
Cholecystokinin plays a key role in **gallbladder contraction** and **pancreatic enzyme secretion**, making it essential for proper digestion and absorption of nutrients.

**Correct Answer:** D. I-cells of duodenum.