**Core Concept**
Diverticulosis is a condition characterized by the formation of diverticula, small pouches or sacs in the wall of the gastrointestinal tract, particularly in the colon. It is a common condition in the elderly population, often associated with a low-fiber diet and increased intraluminal pressure.

**Why the Correct Answer is Right**
The commonest site of diverticulosis is the sigmoid colon, which is the part of the colon that connects the descending colon to the rectum. The sigmoid colon is a narrow, S-shaped portion of the colon that is subject to increased intraluminal pressure, making it more susceptible to the formation of diverticula. The increased pressure and lower fiber intake in the elderly population contribute to the development of diverticulosis in this area.

**Why Each Wrong Option is Incorrect**
* **Option A:** The transverse colon is another part of the colon that can be affected by diverticulosis, but it is less common than the sigmoid colon.
* **Option B:** The cecum is the first part of the large intestine and is not typically associated with diverticulosis.
* **Option D:** The rectum can be affected by diverticulosis, but it is not the most common site.

**Clinical Pearl / High-Yield Fact**
It's essential to note that diverticulosis is often asymptomatic, and symptoms may only occur if the diverticula become inflamed or infected, resulting in diverticulitis. A low-fiber diet and constipation can contribute to the development of diverticulosis, making dietary changes a crucial aspect of prevention.

**Correct Answer: C. Sigmoid colon.**