**Core Concept**
The question is testing the knowledge of the location of gastrin-producing cells in the stomach, which play a crucial role in regulating gastric acid secretion. Gastrin is a peptide hormone that stimulates the parietal cells in the stomach to secrete hydrochloric acid.

**Why the Correct Answer is Right**
The correct answer is the antrum of the stomach, which is the distal part of the stomach. The antrum contains G cells, also known as gastrin-producing cells, which produce gastrin in response to food intake. Gastrin then binds to receptors on the parietal cells in the gastric glands, stimulating them to secrete hydrochloric acid. This process is essential for protein digestion and absorption.

**Why Each Wrong Option is Incorrect**
**Option A:** The fundus of the stomach is the proximal part of the stomach and contains parietal cells that produce gastric acid, but it is not the primary site of gastrin production.
**Option B:** The duodenum is the first part of the small intestine and is not a site of gastrin production.
**Option C:** The pylorus is the region of the stomach that connects to the small intestine and is not a significant site of gastrin production.

**Clinical Pearl / High-Yield Fact**
Gastrin-producing cells in the antrum of the stomach are sensitive to the presence of food, particularly proteins, and respond by releasing gastrin to stimulate gastric acid secretion. This is a crucial adaptation to ensure adequate digestion and absorption of nutrients.

**Correct Answer:** A. Antrum of the stomach.