The waves that requires a medium to travel are called mechanical wave. There are two types of mechanical wave i.e.
(1) Longitudinal wave
(2) Transverse wave
The longitudinal wave travel in the form of compression and rarefaction. In this type, the displacement of medium is in same direction to the direction of propagation of wave. In case of transverse wave, the oscillation of particle is in perpendicular direction.
Hence, the correct option for longitudinal wave is " parallel to the wave motion ".
