**Core Concept**
Helicobacter pylori (H. pylori) is a gram-negative bacterium that inhabits the gastrointestinal tract, particularly in the stomach. It is a significant cause of various gastrointestinal disorders, including peptic ulcers and gastric cancer.

**Why the Correct Answer is Right**
H. pylori predominantly colonizes the gastric antrum, which is the distal portion of the stomach. This region is rich in mucus, providing a suitable environment for the bacterium to adhere to the gastric epithelium and establish a chronic infection. The gastric antrum's acidic environment is also less hostile to H. pylori compared to the more acidic corpus region.

**Why Each Wrong Option is Incorrect**
* **Option A:** H. pylori is not typically found in the duodenum, as its acidic environment is inhospitable to the bacterium.
* **Option B:** The gastric corpus, while involved in acid production, is not the primary site of H. pylori colonization due to its higher acidity.
* **Option D:** The small intestine, including the jejunum and ileum, is not a common site for H. pylori colonization, as its environment is less conducive to the bacterium's survival.

**Clinical Pearl / High-Yield Fact**
It is essential to note that H. pylori infection is a significant risk factor for gastric cancer, and its eradication can reduce this risk. The bacterium's presence can be detected through endoscopy, biopsy, or non-invasive tests such as the urea breath test.

**Correct Answer:** D.