Question

Which of the following is the best definition of longitudinal wave?

In a longitudinal wave, particle displacement is above the direction of wave motion.

In a longitudinal wave, particle displacement is opposite to the direction of wave motion.

In a longitudinal wave, particle displacement is parallel to the direction of wave motion.

In a longitudinal wave, particle displacement is perpendicular to the direction of wave motion.

Answer 1

In a longitudinal wave, particle displacement is parallel to the direction of wave motion.! 

Just remember Looong-itudinal , long = like a spring in a spring its compressions and the wave travels "thru,on the spring" so its parallel to the source/wavemotion. 

the other to note is the last selection , perpendicular is transverse wave. So these 2 are the most common types that will appear :D 
_Brainliest if i helped!