This body part secretes secretin hormone

The correct option is Duodenum.

Step-by-step Explanation:

1. Understanding Secretin:
Secretin is a peptide hormone that plays a crucial role in regulating digestion. Its primary function is to stimulate the pancreas to secrete a fluid rich in bicarbonate, which neutralizes the acidic chyme coming from the stomach into the small intestine.

2. Site of Secretion:
Secretin is produced and released by specific endocrine cells called S cells. These S cells are located predominantly in the mucosa of the duodenum, which is the first and shortest segment of the small intestine.

3. Mechanism of Stimulus:
When acidic gastric contents (containing hydrochloric acid) pass from the stomach into the duodenum, the drop in pH (specifically below 4.5) triggers the S cells in the duodenal mucosa to secrete secretin into the bloodstream.

4. Evaluating Other Options:
- Oesophagus: The oesophagus acts primarily as a conduit for food transport from the mouth to the stomach and does not secrete digestive hormones.
- Stomach: The stomach secretes hormones like gastrin, but not secretin.
- Ileum: The ileum is the final section of the small intestine; while it is involved in absorption and secretes some local peptides, secretin is primarily synthesized and released in the duodenum.