**Core Concept**
The question is testing the most effective and cost-efficient screening method for a particular condition. The underlying principle is to identify a method that can detect the condition at an early stage, is widely available, and has a high sensitivity and specificity.

**Why the Correct Answer is Right**
The most economical and best screening method is the **Fecal Occult Blood Test (FOBT)**. This test detects hidden blood in the stool, which can indicate colorectal cancer or other gastrointestinal conditions. FOBT is a simple and non-invasive test that can be performed at home or in a clinic. It involves applying a small amount of feces to a test strip, which is then checked for blood. If the result is positive, further diagnostic tests, such as colonoscopy, may be required.

**Why Each Wrong Option is Incorrect**
**Option A:** Other options, such as sigmoidoscopy or colonoscopy, are more invasive and expensive than FOBT.
**Option B:** While endoscopy is an effective diagnostic tool, it is not the most economical screening method due to its higher cost and requirement for specialized equipment and expertise.
**Option C:** CT colonography is a more expensive and resource-intensive test than FOBT, making it a less economical option for screening.

**Clinical Pearl / High-Yield Fact**
The American Cancer Society recommends that adults aged 50 and older undergo regular FOBT screening for colorectal cancer.

**Correct Answer:** D. Fecal Occult Blood Test (FOBT)