**Core Concept**
The question is testing the understanding of the anatomy of the hernia sac and its relationship with the intestine. Specifically, it is inquiring about the portion of the circumference of the intestine that is contained within the sac.

**Why the Correct Answer is Right**
The hernia sac typically contains a portion of the circumference of the intestine, known as the herniated bowel loop. This occurs when the peritoneal sac protrudes through a weakened area in the abdominal wall, carrying a portion of the intestine with it. The hernia sac usually contains a segment of the intestine that is not fixed to the surrounding structures, allowing it to bulge through the defect.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the hernia sac does not contain the entire circumference of the intestine. Instead, it typically contains a portion of the circumference.

**Option B:** This option is incorrect because the hernia sac does not contain the entire length of the intestine. The herniated bowel loop is usually a segment of the intestine, not the entire length.

**Option C:** This option is incorrect because the hernia sac does not contain the entire thickness of the intestine. The herniated bowel loop is usually a segment of the intestinal wall, not the entire thickness of the intestine.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the hernia sac contains only a portion of the circumference of the intestine, which is why hernias can be reducible or irreducible. Reducible hernias can be pushed back into the abdominal cavity, while irreducible hernias cannot be pushed back due to the presence of the herniated bowel loop.

**Correct Answer: D.**