**Core Concept**
The colon is a part of the large intestine with a distinct anatomical feature that differentiates it from other parts of the gastrointestinal tract. Specifically, the colon has a varying degree of peritoneal attachment, which is reflected in the presence or absence of a mesentery.

**Why the Correct Answer is Right**
The correct answer refers to the part of the colon that lacks a mesentery. This is due to its retroperitoneal location, where it is fixed to the posterior abdominal wall by fibrous connections. Unlike the other parts of the colon, which are suspended by a mesentery, the part without a mesentery is anchored firmly in place. This anatomical arrangement has implications for surgical approaches and potential complications.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because the ascending colon is actually retroperitoneal, but it does have a mesentery. The correct answer refers to a part of the colon that lacks a mesentery.
* **Option B:** This is incorrect because the transverse colon is suspended by a mesentery, known as the transverse mesocolon. It does not lack a mesentery.
* **Option C:** This is incorrect because the descending colon has a mesentery, known as the mesosigmoid, which attaches it to the pelvic sidewall.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the part of the colon without a mesentery is the ascending colon, which can be a critical consideration during surgical procedures, particularly those involving the retroperitoneal space.

**Correct Answer: A. Ascending colon**