The correct answer is option 4, which states that the frequency of ultrasound waves is greater than 20 kHz.

Ultrasound waves are a type of mechanical wave that has a frequency higher than the range of human hearing, which is typically between 20 Hz and 20 kHz. Option 1 states that the frequency is less than 20 Hz, which is incorrect because ultrasound waves have a much higher frequency than that. Option 2 states that the frequency is between 20 Hz and 2 kHz, which is also incorrect as it falls within the audible range rather than the ultrasound range. Option 3 states that the frequency is between 2 kHz and 20 kHz, which is also incorrect as it still falls within the audible range and not the ultrasound range.

Therefore, the correct option is option 4, which states that the frequency of ultrasound waves is greater than 20 kHz. Ultrasound waves with their high frequency are used in various applications such as medical imaging, industrial testing, and cleaning processes.