Question

if the molecules of a body move ____ to the direction in which a wave travels, the wave is called a longitudinal wave.

Explanation:

In a longitudinal wave, the particles (molecules) of the medium vibrate parallel to the direction of wave propagation. This is a key characteristic that distinguishes longitudinal waves (like sound waves in air) from transverse waves (where particles vibrate perpendicular to the wave direction). So the correct word to fill in the blank is "parallel".

Answer:

parallel