**Core Concept**
Ultrasound (USG) technology relies on the principle of echolocation, where high-frequency sound waves are emitted into the body and the echoes are received to create images. This process is based on the interaction between sound waves and tissue interfaces, allowing for real-time imaging.

**Why the Correct Answer is Right**
The correct answer is based on the piezoelectric effect, where crystals vibrate when exposed to an electric current, producing high-frequency sound waves. These sound waves interact with tissue interfaces, causing the echoes to be reflected back to the transducer, which then converts the echoes into electrical signals. The electrical signals are processed to create two-dimensional images of internal structures. The Doppler shift phenomenon is also utilized in USG to detect movement, such as blood flow.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because USG does not rely on X-rays or ionizing radiation.
**Option B:** Incorrect because USG does not use electromagnetic waves, such as those used in MRI or CT scans.
**Option C:** Incorrect because USG does not use radioactive isotopes or nuclear medicine principles.

**Clinical Pearl / High-Yield Fact**
USG is a non-invasive, real-time imaging modality that does not use ionizing radiation, making it an essential tool in various medical specialties, including obstetrics, cardiology, and emergency medicine.

**Correct Answer: C. Ultrasound (USG) is based on the principle of echolocation and the piezoelectric effect.**