Answer:
C. undulating displacement of air molecules caused by pressure changes.
Explanation:
Sound is a mechanical wave arising from the movement back and forth of the media objects through which the sound wave travels.If a sound wave moves through air from left to right, then particles of air will move both to the right and to the left as the sound wave's energy passes through it.
The sound wave is formed by air molecules that are displaced due to pressure changes.
Therefore the answer is C.
