This is a wave whose energy travels in the same direction as the energy being transferred.

LONGITUDINAL WAVE

Explanation:

As we know that there are two type of propagation of wave

1) Transverse wave :

in transverse nature of waves the energy is propagated into the medium in perpendicular to the wave propagation.

So here in transverse waves the medium molecules will start oscillating in perpendicular direction

2) Longitudinal wave

Here in this type of wave the medium molecules will start oscillating in the direction of propagation of waves. So here the medium molecules will oscillate in the direction parallel to the wave propagation

so correct answer is

Longitudinal waves

a and b sometimes have a positive value