**Core Concept**
Colon carcinoma is a type of malignant tumor that arises from the epithelial cells lining the colon. Early detection and diagnosis are crucial for effective treatment and improved patient outcomes.

**Why the Correct Answer is Right**
The best investigation for carcinoma colon is **Colonoscopy**. This procedure allows for direct visualization of the colon mucosa, enabling the detection of polyps, tumors, and other abnormalities. A colonoscopy also provides the opportunity for biopsy and removal of polyps, which can help prevent the development of colorectal cancer.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan of the abdomen. While a CT scan can help identify large tumors and metastatic disease, it is not as sensitive as colonoscopy for detecting small polyps and early-stage cancer.

**Option B:** Barium enema. Although a barium enema can help visualize the colon and detect abnormalities, it is less sensitive than colonoscopy and does not allow for direct visualization or biopsy of lesions.

**Option C:** MRI of the abdomen. MRI can provide detailed images of the abdominal organs, but it is not the best investigation for detecting colon carcinoma due to its lower sensitivity compared to colonoscopy.

**Clinical Pearl / High-Yield Fact**
The American Cancer Society recommends that adults with an average risk of colorectal cancer undergo regular screening starting at age 45, using a test such as colonoscopy every 10 years.

**Correct Answer: B. Barium enema is not the best investigation for carcinoma colon.