**Core Concept**
Ulcerative colitis (U.C.) is a type of inflammatory bowel disease (IBD) characterized by chronic inflammation and ulceration of the colon mucosa. The disease primarily affects the colon, but the extent of involvement can vary.

**Why the Correct Answer is Right**
The correct answer is the part of the colon most commonly affected in ulcerative colitis. This is due to the characteristic feature of the disease, which is inflammation starting from the rectum and extending proximally in a continuous manner. The rectum is the most commonly affected site because of its direct exposure to fecal stream, which contains bacteria that can trigger an immune response. The inflammation in the rectum can lead to symptoms such as rectal bleeding, diarrhea, and urgency.

**Why Each Wrong Option is Incorrect**
* **Option A:** While the sigmoid colon can be involved in ulcerative colitis, it is not the most commonly affected area.
* **Option B:** The right colon is less commonly affected in ulcerative colitis, as the disease typically starts in the left colon and extends proximally.
* **Option D:** The transverse colon can be affected in ulcerative colitis, but it is not the most commonly affected area.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that ulcerative colitis typically starts in the rectum and extends proximally in a continuous manner, making rectal involvement a hallmark of the disease.

**Correct Answer:** C.