**Core Concept**
The most useful investigation for Vesicovaginal Fistula (VVF) is one that provides a clear visualization of the fistula's location, size, and relationship to surrounding structures. VVF is an abnormal connection between the bladder and vagina, often resulting from childbirth or surgical complications.

**Why the Correct Answer is Right**
The correct answer is **Cervicogram**. A cervicogram is a type of X-ray that uses a contrast agent to visualize the cervix and vagina. It is particularly useful for diagnosing VVF because it can show the fistula's location and size, as well as any associated pelvic abnormalities. The cervicogram is often performed in conjunction with a voiding cystourethrogram (VCUG) to assess the bladder's function and any associated ureteral obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Cystoscopy is a procedure that visualizes the inside of the bladder and urethra, but it may not provide a clear view of the fistula's location or size, especially if it is located near the cervix.

**Option B:** Ultrasound is a useful imaging modality for many gynecological conditions, but it may not provide sufficient detail to diagnose VVF, especially if the fistula is small or located in a difficult-to-image area.

**Option D:** MRI is a highly sensitive imaging modality that can provide detailed images of the pelvic organs, but it is often more expensive and less readily available than other imaging options, making it less practical for initial diagnosis of VVF.

**Clinical Pearl / High-Yield Fact**
A key factor in the diagnosis and treatment of VVF is the timing of the investigation. A delayed diagnosis can lead to increased risk of complications, including chronic urinary incontinence and pelvic pain.

**Correct Answer:** C. Cervicogram.