Answer:
Parallel to the direction of wave movement
Step-by-step explanation:
In a longitudinal wave :
The displacement of particle is parallel to the direction of wave movement in the longitudinal wave.
These wave are produce in the form of compression and rarefaction when travelling through the medium.
In transverse wave :
The displacement of particle is perpendicular to the direction of wave movement in the transverse wave.
Hence, Parallel to the direction of wave movement.