(b) higher frequency
Ultrasound waves have higher frequency than audible sound waves.
The audible range of sound for human beings is from 20 to 20000 Hz. Children under the age of 5 and more animals such as dogs can hear upto 25000 Hz. As people grow older, their ears become less sensitive to higher frequency. Ultrasonic sound of frequency higher than 20000 Hz, which cannot be heard by human beings.
