**Core Concept**
The investigation of choice for diagnosing carcinoma colon involves imaging modalities that can accurately visualize the colon and its contents. This requires a modality that can provide detailed images of the colon lining, detect any abnormalities, and guide further management.

**Why the Correct Answer is Right**
The correct answer is colonoscopy, which is a procedure that uses a flexible tube with a camera and light on the end to visualize the entire colon. This allows for the detection of polyps, tumors, and other abnormalities, and biopsies can be taken during the procedure to confirm the diagnosis. Colonoscopy is considered the gold standard for diagnosing colon cancer because it can visualize the entire colon, detect precancerous polyps, and remove them during the procedure to prevent cancer development.

**Why Each Wrong Option is Incorrect**
* **Option A:** Barium enema is an older imaging modality that involves inserting a barium solution into the colon through the rectum, followed by X-rays to visualize the colon. While it can detect some abnormalities, it is less sensitive than colonoscopy and cannot provide a detailed view of the colon lining.
* **Option B:** CT colonography, also known as virtual colonoscopy, uses a CT scanner to create detailed images of the colon. However, it requires bowel prep and may not be as sensitive as colonoscopy for detecting small polyps or flat lesions.
* **Option D:** MRI colonography is a newer imaging modality that uses magnetic resonance imaging to visualize the colon. However, it is not widely available and may not be as sensitive as colonoscopy for detecting colon cancer.

**Clinical Pearl / High-Yield Fact**
Colonoscopy is not only used for diagnosing colon cancer but also for screening high-risk individuals and removing precancerous polyps to prevent cancer development.

**Correct Answer: C. Colonoscopy**