**Core Concept**
Perforation during colonoscopy most commonly occurs in the sigmoid colon due to its anatomical location, relatively narrow lumen, and the presence of high intraluminal pressure during insertion, especially in patients with diverticular disease or strictures.

**Why the Correct Answer is Right**
The sigmoid colon is the most frequently involved site of perforation during colonoscopy. This is because it has a narrow lumen, is more prone to mucosal trauma, and is often the last segment to be visualized during the procedure. The sigmoid colon is also more susceptible to perforation due to its proximity to the rectum and the high pressure exerted during insertion, especially in patients with pre-existing pathology like diverticulosis. The risk increases with longer insertion time and inadequate bowel preparation.

**Why Each Wrong Option is Incorrect**
Option A: The caecum is the most distal and broadest segment of the colon and has a larger lumen, making it less prone to perforation. While it can be involved, it is not the most common site.
Option B: The hepatic flexure is a sharp bend with minimal lumen and is not routinely reached during colonoscopy, reducing the risk of injury.
Option C: The splenic flexure is a narrow, rigid segment, but it is not the most common site of perforation due to limited exposure and less pressure during insertion.

**Clinical Pearl / High-Yield Fact**
Perforation during colonoscopy is rare but serious; the sigmoid colon is the most common site, and patients with known diverticular disease or prior colonic surgery are at higher risk. Always ensure adequate bowel prep and avoid excessive force during insertion.

✓ Correct Answer: D. Sigmoid colon