**Core Concept**
The investigation of choice for small intestine tumors involves imaging modalities that can visualize the small intestine in detail, as these tumors are often subtle and may not be palpable. The small intestine is a long, thin, and tortuous structure, making it challenging to visualize with standard imaging techniques.

**Why the Correct Answer is Right**
The correct investigation of choice for small intestine tumors is **CT Enterography**. This involves the use of a contrast agent that is absorbed by the intestinal mucosa, allowing for clear visualization of the small intestine's lumen and wall. CT Enterography is particularly useful for detecting tumors, as it can identify subtle wall thickening and luminal narrowing. The use of a thin slice thickness (typically 1-2 mm) and high-resolution reconstruction algorithms allows for excellent visualization of the small intestine's mucosal and submucosal layers.

**Why Each Wrong Option is Incorrect**
**Option A:** **Barium Swallow** - While a barium swallow can visualize the upper part of the small intestine, it is not the best modality for small intestine tumors due to its inability to clearly visualize the distal small intestine and the potential for barium to obscure small lesions.

**Option B:** **MRI Enterography** - While MRI can provide excellent soft tissue contrast and is useful for detecting small intestine tumors, it is not as widely available as CT and may be more expensive.

**Option C:** **Colonoscopy** - Colonoscopy is primarily used for examining the colon and rectum and is not suitable for visualizing the small intestine.

**Clinical Pearl / High-Yield Fact**
Small intestine tumors are often asymptomatic in the early stages and may present with non-specific symptoms such as abdominal pain, weight loss, and gastrointestinal bleeding. A high index of suspicion is required for early detection and treatment.

**Correct Answer:** C.