**Core Concept**
Piezoelectric crystals are materials that generate an electric charge in response to mechanical stress, such as pressure or vibration. This property is utilized in various medical devices, including ultrasonic probes used for diagnostic imaging. The most commonly used piezoelectric crystal in medical devices is a specific type of material that exhibits high sensitivity and stability.

**Why the Correct Answer is Right**
The most commonly used piezoelectric crystal is Lead Zirconate Titanate (PZT). PZT is a ferroelectric material that exhibits a high piezoelectric coefficient, making it an ideal choice for medical ultrasonic devices. When subjected to mechanical stress, PZT generates a significant electric charge, which is then converted into an ultrasonic signal used for imaging. The high sensitivity and stability of PZT make it an essential component in modern ultrasonic probes.

**Why Each Wrong Option is Incorrect**

* **Option A:** Lithium Niobate is a piezoelectric material, but it is not as commonly used as PZT in medical ultrasonic devices due to its higher cost and lower sensitivity.
* **Option B:** Quartz is a piezoelectric material, but it has a lower piezoelectric coefficient compared to PZT, making it less suitable for high-frequency ultrasonic applications.
* **Option C:** Barium Titanate is a ferroelectric material, but it is not as widely used as PZT in medical ultrasonic devices due to its lower sensitivity and stability.

**Clinical Pearl / High-Yield Fact**
When selecting a piezoelectric crystal for medical ultrasonic devices, it is essential to consider factors such as sensitivity, stability, and cost. PZT is the most commonly used piezoelectric crystal due to its high sensitivity and stability, making it an ideal choice for high-frequency ultrasonic applications.

**Correct Answer:** C. Barium Titanate