**Core Concept**
Meckel's diverticulum is a congenital anomaly of the small intestine, resulting from the incomplete closure of the vitelline duct. It is a true diverticulum, containing all layers of the intestinal wall. When a Meckel's diverticulum is found within an inguinal hernia sac, it is a significant finding, often requiring surgical intervention.

**Why the Correct Answer is Right**
The presence of a Meckel's diverticulum within an inguinal hernia sac is referred to as a Littre's hernia. This occurs when a Meckel's diverticulum becomes herniated through a weakness in the abdominal wall, such as a inguinal hernia. The vitelline duct, from which the Meckel's diverticulum arises, is a remnant of the yolk sac. Littre's hernia is a rare but important condition to recognize, as it can lead to complications such as intestinal obstruction or diverticulitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the question, as it does not pertain to the specific scenario of a Meckel's diverticulum within an inguinal hernia sac.
**Option B:** This option is incorrect, as it refers to a different condition altogether. A hernia containing a Meckel's diverticulum is specifically called a Littre's hernia, not a Richter's hernia.
**Option C:** This option is incorrect, as it refers to a condition where a Meckel's diverticulum is found in the abdominal wall, but not within an inguinal hernia sac.

**Clinical Pearl / High-Yield Fact**
A Littre's hernia is a rare but significant condition that requires prompt surgical attention. It is essential to recognize the presence of a Meckel's diverticulum within an inguinal hernia sac to prevent complications.

**Correct Answer: C. Littre's hernia.**