Which is not true of ultrasound?
a. it is a mechanical longitudinal wave
b. it is sound lower in frequency than that which is audible to humans
c. it requires a medium in which to travel
d. its attenuation increases as path length and frequency increase

Hi,

Choice 'b' is the correct answer to your question.

"The term "ultrasonic" applied to sound refers to anything above the
frequencies of audible sound, and nominally includes anything over
20,000 Hz. Frequencies used for medical diagnostic ultrasound scans
extend to 10 MHz and beyond..."
Ultrasonic Sound
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/usound.html

Thank you for your question.

Warmest regards,
Shiv Reddy