**Core Concept**
Brunner's glands are a type of mucous gland located in the duodenum, playing a crucial role in the digestive process by secreting alkaline mucous that helps neutralize the acidic chyme from the stomach.

**Why the Correct Answer is Right**
The correct answer is B. Upper duodenum because Brunner's glands are specifically found in the submucosa of the upper part of the duodenum, the first segment of the small intestine. These glands are rich in mucous cells that produce an alkaline secretion, which helps protect the duodenal mucosa from the acidic gastric juice. This neutralization process creates a favorable environment for the absorption of nutrients.

**Why Each Wrong Option is Incorrect**
**Option A:** The jejunum, the middle segment of the small intestine, does not contain Brunner's glands. The jejunum is primarily responsible for the absorption of nutrients.
**Option C:** The lower duodenum does not have a significant number of Brunner's glands, as they are primarily located in the upper part of the duodenum.
**Option D:** The appendix is a small, tube-like structure attached to the large intestine and does not contain Brunner's glands.

**Clinical Pearl / High-Yield Fact**
Brunner's glands are an essential component of the duodenal mucosa, and their secretion plays a vital role in maintaining the integrity of the duodenal lining. This is a key concept to remember, especially when considering the pathophysiology of duodenal ulcers and other gastrointestinal disorders.

**✓ Correct Answer: B. Upper duodenum**