**Core Concept**
Colonoscopy is a diagnostic tool used to visualize the interior of the colon and rectum, allowing for the detection of polyps, tumors, and other abnormalities. It is a crucial investigation in the evaluation of colorectal cancer.

**Why the Correct Answer is Right**
Colonoscopy is the preferred method for the investigation of colon cancer due to its ability to visualize the entire colon and rectum, allowing for the detection of lesions and biopsies. This is achieved through the insertion of a flexible tube with a camera and light on the end, which is guided through the anus and into the colon. The procedure also enables the removal of polyps, which can help prevent the development of cancer. The high sensitivity and specificity of colonoscopy make it the gold standard for the investigation of colon cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is useful for staging colon cancer but is not the primary investigation for colon cancer diagnosis.
**Option B:** Barium enema is a less sensitive and less specific method for colon cancer diagnosis compared to colonoscopy.
**Option D:** X-ray is not suitable for the investigation of colon cancer due to its limited ability to visualize the colon and rectum.

**Clinical Pearl / High-Yield Fact**
Colonoscopy is not only a diagnostic tool but also a therapeutic one, allowing for the removal of polyps and prevention of colon cancer. Regular screening with colonoscopy can significantly reduce the incidence and mortality of colon cancer.

**✓ Correct Answer: C. Colonoscopy**