**Core Concept**
The question is testing the understanding of the pathophysiology of gallstone ileus, a mechanical bowel obstruction caused by a gallstone that has migrated from the biliary system into the small intestine. This occurs when a gallstone erodes through the gallbladder wall into the adjacent duodenum, allowing it to enter the intestinal lumen.

**Why the Correct Answer is Right**
The correct answer is **B. Proximal to iliocaecal junction** because the most common site of gallstone impactation is at the terminal ileum, which is proximal to the iliocaecal junction. This occurs due to the narrowing of the ileum at this point, which causes the gallstone to become lodged. The terminal ileum is the narrowest part of the small intestine, making it more susceptible to gallstone impaction. The gallstone is usually too large to pass through the ileocecal valve, which is located at the iliocaecal junction.

**Why Each Wrong Option is Incorrect**
**Option A:** The duodenojejunal junction is not a common site for gallstone impaction. While gallstones can erode into the duodenum, they usually continue to migrate through the small intestine until they become lodged at a narrower point, such as the terminal ileum.
**Option C:** The distal to iliocaecal junction option is incorrect because gallstones typically become lodged before reaching the iliocaecal junction. The narrowing of the ileum proximal to this point, combined with the size and shape of the gallstone, usually causes the impaction.
**Option D:** The colon is not a common site for gallstone impaction. While gallstones can sometimes bypass the ileocecal valve and enter the colon, they are more likely to become lodged in the small intestine, particularly at the terminal ileum.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that gallstone ileus is a mechanical bowel obstruction, not an inflammatory or ischemic one. Therefore, surgical intervention is often necessary to relieve the obstruction and remove the impacted gallstone.

**✓ Correct Answer: B. Proximal to iliocaecal junction**