**Core Concept**
Intravenous cholecystography (IVCG) is a diagnostic imaging technique used to visualize the gallbladder and its contents. The underlying principle involves the use of a contrast agent that is taken up by the gallbladder, allowing for radiographic visualization of gallstones.

**Why the Correct Answer is Right**
The correct answer is Ultrasonography (USG). Ultrasonography is the investigation of choice for gallbladder stones due to its non-invasive nature, lack of ionizing radiation, and high sensitivity for detecting gallstones. It can also assess the size, number, and location of gallstones, as well as the thickness of the gallbladder wall. The ultrasound waves are able to penetrate the body and bounce off the gallstones, producing an image that can be used for diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** CT Scan is not the investigation of choice for gallbladder stones as it exposes the patient to ionizing radiation and is more expensive than ultrasonography.
* **Option B:** MRI is not commonly used for diagnosing gallbladder stones due to its high cost and limited availability.
* **Option D:** Endoscopic retrograde cholangiopancreatography (ERCP) is an invasive procedure used for therapeutic purposes, such as removing gallstones or treating bile duct obstruction, but is not used for initial diagnosis.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that ultrasonography is the first-line imaging modality for suspected gallbladder disease, as it is non-invasive, cost-effective, and provides valuable information for diagnosis and management.

**Correct Answer: C. Ultrasonography (USG)