**Core Concept**
The colon is a part of the large intestine, and like other abdominal organs, it has a mesentery that attaches it to the back of the abdominal wall. However, there is a specific part of the colon that lacks a mesentery, making it unique in terms of its anatomical attachment.

**Why the Correct Answer is Right**
The part of the colon without a mesentery is the **rectum**. The rectum is the final portion of the large intestine, extending from the sigmoid colon to the anal canal. Unlike other parts of the colon, it does not have a mesentery, which is a fold of peritoneum that attaches the colon to the abdominal wall. This is because the rectum is located within the pelvis and does not require a mesentery for its attachment.

**Why Each Wrong Option is Incorrect**
* **Option A:** The transverse colon, which is a part of the colon that does have a mesentery, attaches to the abdominal wall via the transverse mesocolon.
* **Option B:** The sigmoid colon, which is a part of the colon that has a mesentery, attaches to the abdominal wall via the sigmoid mesocolon.
* **Option C:** The ascending and descending colon, which are parts of the colon that have a mesentery, also attach to the abdominal wall via their respective mesenteries.

**Clinical Pearl / High-Yield Fact**
The absence of a mesentery in the rectum makes it more susceptible to injuries and complications during surgical procedures, such as rectal cancer resections or pelvic fractures.

**Correct Answer: C. Ascending and Descending Colon.