**Core Concept**
Brunner's glands are specialized submucosal glands that play a crucial role in the production of alkaline mucus in the gastrointestinal tract. They are an essential part of the protective mechanism against acidic chyme from the stomach, secreting a bicarbonate-rich fluid to neutralize the pH.

**Why the Correct Answer is Right**
The correct location of Brunner's glands is the duodenum, the first segment of the small intestine. This is due to their specific function in neutralizing the acidic chyme from the stomach, which enters the duodenum through the pyloric sphincter. The bicarbonate-rich secretions of Brunner's glands help to create an optimal environment for pancreatic enzymes to function effectively.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because Brunner's glands are not located in the stomach, where the acidic chyme originates. The stomach itself has its own mucous glands, but they are not the same as Brunner's glands.

**Option B:** This is incorrect because the small intestine, while a part of the gastrointestinal tract, is a broader term that encompasses multiple segments beyond the duodenum. While Brunner's glands do secrete their mucus here, the specific location is more precise.

**Option C:** This is incorrect because the jejunum, another segment of the small intestine, does not have Brunner's glands. The jejunum is more distal to the duodenum and has different specialized functions.

**Option D:** This is incorrect because the ileum, the final segment of the small intestine, does not have Brunner's glands. The ileum is involved in the absorption of bile salts and vitamin B12, but it does not require the specific protective mechanism provided by Brunner's glands.

**Clinical Pearl / High-Yield Fact**
Brunner's glands are an essential component of the duodenal mucosa, and their secretions are critical in maintaining the pH balance necessary for optimal pancreatic enzyme function. Remember that Brunner's glands are specifically adapted to the duodenum, making this the correct location.

**Correct Answer:** D. Ileum