**Core Concept**
The question is testing the student's knowledge of the anatomy of the abdominal cavity, specifically the relationship between the viscera and the peritoneal cavity. The peritoneal cavity is a space between the parietal and visceral peritoneum that contains the abdominal organs.

**Why the Correct Answer is Right**
The correct answer is related to the concept of the visceral peritoneum, which is a layer of the peritoneum that covers the abdominal viscera. The visceral peritoneum is in direct contact with the abdominal organs and forms the wall of the omental bursa (also known as the lesser sac), which is a cavity located behind the stomach and the liver. The visceral peritoneum also forms the wall of the pelvic cavity, which contains the pelvic organs.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the parietal peritoneum forms the wall of the peritoneal cavity, not the visceral peritoneum.
* **Option B:** This option is incorrect because the diaphragm separates the thoracic and abdominal cavities, but it is not formed by the visceral peritoneum.
* **Option C:** This option is incorrect because the mesentery is a fold of the peritoneum that attaches the intestine to the back of the abdominal wall, but it is not formed by the visceral peritoneum.

**Clinical Pearl / High-Yield Fact**
The peritoneum is a double-layered membrane that lines the abdominal cavity and covers the abdominal organs. The parietal peritoneum lines the abdominal wall, while the visceral peritoneum covers the abdominal viscera.

**Correct Answer: D. The omental bursa (lesser sac)