Correct Answer (Detailed Explanation is Below)
C.
Ultrasonic sound
Detailed Explanation
Ultrasonic sound has a frequency greater than 20,000 Hz, which is generally beyond normal human hearing. Ultrasound is widely used in
medical imaging, SONAR, industrial testing and cleaning devices. In contrast, infrasonic waves have frequencies below 20 Hz. Both are mechanical waves and generally require a material medium for propagation.