Question

sound waves are an example of.
longitudinal waves
electromagnetic waves
waves that can travel through a vacuum
waves with visible crests and troughs

Explanation:

Sound waves are longitudinal. In longitudinal waves, particle displacement is parallel to wave direction. Electromagnetic waves (like light) can travel in vacuum; sound needs medium. Transverse waves (not sound) have visible crests/troughs. So sound is longitudinal.

Answer:

Longitudinal waves