Question

longitudinal waves move in a ------ direction.

Explanation:

In physics (a subfield of Natural Science), longitudinal waves are defined as waves where the particle displacement is parallel to the direction of wave propagation. So the direction of longitudinal waves' movement (and particle oscillation) is parallel to the wave's travel direction, often described as "parallel" or "same" as the wave's propagation. But the key concept here is that longitudinal waves move in a direction parallel to the direction of particle vibration (or the wave's propagation), and the standard term to fill in is "parallel" (or sometimes "same" but "parallel" is more precise in physics for the direction of wave motion relative to particle displacement).

Answer:

parallel