**Core Concept**
Colon cancer diagnosis requires a method that can accurately visualize the entire colon, detect polyps and tumors, and guide biopsies. The ideal investigation should be non-invasive, relatively quick, and cost-effective.

**Why the Correct Answer is Right**
Colonoscopy is the investigation of choice for diagnosing colon cancer. It involves the insertion of a flexible tube with a camera and light on the end into the colon through the rectum. This allows the physician to visually inspect the entire colon, identify polyps and tumors, and perform biopsies to confirm the diagnosis. The procedure is typically performed under sedation and takes about 30-60 minutes. Colonoscopy has a high sensitivity and specificity for detecting colon cancer and is considered the gold standard for screening and diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** CT Colonography (Virtual Colonoscopy) - While CT Colonography is a useful screening tool, it is not as sensitive as colonoscopy for detecting small polyps and tumors. It is also not suitable for guiding biopsies.

**Option B:** Barium Enema - This is an older radiographic technique that uses a barium sulfate contrast agent to outline the colon. However, it is less sensitive than colonoscopy and cannot guide biopsies.

**Option C:** MRI Colonography - MRI Colonography is a newer technique that uses magnetic resonance imaging to visualize the colon. However, it is not widely available and is not as sensitive as colonoscopy for detecting colon cancer.

**Clinical Pearl / High-Yield Fact**
The American College of Gastroenterology recommends that adults aged 50-75 undergo regular colon cancer screening with colonoscopy every 10 years. Early detection and removal of polyps can prevent colon cancer.

**Correct Answer: C. Colonoscopy is the investigation of choice for diagnosing colon cancer.**