<prompt> The ultrasonic sound waves employed in ultrasound machine for medical sonography have the frequency of2 - 20 MHzLess than 1 MHz20-20,000 HzGreater than 100 MHz  <response>: Frequency refers to a characteristic of the sound that is, the number of times that conduction paicles vibrate back and foh per second. Frequency of sound waves is usually between 1 and 20 MHz in medical sonography. Hence the best answer for this question would be A. 2-20 MHz Christensens Physics of Diagnostic Radiology, 4E,Pg no 347 