Assertion: Ultrasonic waves are longitudinal waves of frequency greater than 20,000 Hz.
Reason: The maximum frequency of audible sound waves is 20,000 Hz.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct and Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Assertion is incorrect but Reason is correct

Hint: Ultrasonic waves are longitudinal waves of frequency greater than 20,000 Hz. This implies that a human being cannot hear beyond 20,000 Hz, i.e. the maximum frequency of audible sound is 20,000 Hz. This will help approach the solution.

Complete step-by-step solution -
Ultrasound refers to an acoustic wave with a frequency greater than the upper limit of human hearing, which is usually over 20 kHz.
Humans have the ability to hear sound waves of frequency between 20 Hz and 20 kHz.
Any sound with a frequency higher than 20,000 hertz (20 kHz = 20,000 cycles of oscillation per second) cannot be sensed by a human's ear. Among audible sounds not above 20 kHz, those not intended to be heard by humans have also come to be called ultrasound.

So option A is the answer that is Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

Note: Sound with a frequency above 20,000 Hz (or 20 kHz)—that is, above the highest audible frequency—is defined to be ultrasound. In practice, it is possible to create ultrasound frequencies up to more than a gigahertz. (Higher frequencies are difficult to create; furthermore, they propagate poorly because they are very strongly absorbed.) Ultrasound has a tremendous number of applications, which range from burglar alarms and it is also used to clean sensitive objects, moreover it provides guidance systems for aerospace. We begin our discussion of ultrasound with a number of its applications in medicine, during which it's used extensively both for diagnosis and for therapy.