**Core Concept**
Choledochal cysts are congenital anomalies of the bile ducts, characterized by dilation of the intrahepatic or extrahepatic bile ducts. The investigation of choice for diagnosing choledochal cysts should ideally be non-invasive, accurate, and able to provide detailed information about the cyst's location, size, and shape.

**Why the Correct Answer is Right**
Magnetic Resonance Cholangiopancreatography (MRCP) is the preferred imaging modality for diagnosing choledochal cysts. MRCP uses a combination of magnetic resonance (MR) imaging and a contrast agent to create detailed images of the bile ducts, allowing for the identification of cystic dilatations and other abnormalities. The high-resolution images provided by MRCP enable accurate diagnosis and preoperative planning.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ultrasound is a useful screening tool for detecting choledochal cysts, but it may not provide sufficient detail for accurate diagnosis, particularly in cases of complex cystic dilatations.
* **Option B:** Computed Tomography (CT) scans can provide detailed images of the bile ducts, but they may not be as sensitive as MRCP for detecting small or complex cysts.
* **Option D:** Endoscopic Retrograde Cholangiopancreatography (ERCP) is a therapeutic procedure that involves inserting a catheter into the bile ducts to diagnose and treat abnormalities, but it is not typically used as a first-line diagnostic tool for choledochal cysts.

**Clinical Pearl / High-Yield Fact**
MRCP is particularly useful for diagnosing choledochal cysts because it allows for the identification of the Todani classification, which is essential for preoperative planning and surgical decision-making.

**Correct Answer:** C. MRCP.