**Core Concept**
The lesser sac, also known as the omental bursa, is a peritoneal cavity located behind the stomach and the liver. It is a small, narrow space that is bounded by several anatomical structures.

**Why the Correct Answer is Right**
The lesser sac is indeed bounded by the posterior wall of the stomach, the visceral surface of the spleen, and the under surface of the liver. The posterior wall of the stomach forms the anterior boundary of the lesser sac, while the visceral surface of the spleen forms its left boundary. The under surface of the liver forms the right and superior boundaries of the lesser sac. The correct answer reflects the accurate anatomical boundaries of the lesser sac.

**Why Each Wrong Option is Incorrect**
**Option A:** The posterior wall of the stomach is indeed an anatomical boundary of the lesser sac, but it is not the only one. However, the question asks for all the boundaries, so option A is incomplete.
**Option B:** The visceral surface of the spleen is a boundary of the lesser sac, but it is not the only one. The spleen is located on the left upper quadrant of the abdomen and is not directly adjacent to the liver.
**Option C:** The under surface of the liver is indeed a boundary of the lesser sac, but it is not the only one. The liver is a large organ that covers most of the upper part of the abdomen, and its under surface forms the right and superior boundaries of the lesser sac.
**Option D:** The greater omentum is a fold of peritoneum that hangs from the greater curvature of the stomach and the proximal part of the duodenum. It does not form a boundary of the lesser sac.

**Clinical Pearl / High-Yield Fact**
The lesser sac is an important anatomical landmark in abdominal surgery, as it contains several vital structures, including the pancreas, the splenic vessels, and the hepatic arteries. Surgeons must be aware of the boundaries of the lesser sac to avoid damaging these structures during surgical procedures.

**Correct Answer:** C. Under surface of liver