Concept:A longitudinal wave has alternating regions where particles are either bunched together or spread apart.
Explanation:In a longitudinal wave, particles vibrate parallel to the direction in which the wave travels.
This vibration produces a series of alternate regions along the wave.
The compressed regions, where particles are crowded closely together, are called compressions.
The spaced-out regions, where particles are pulled far apart, are called rarefactions.
These two regions always appear together in a longitudinal wave, such as a sound wave.
Hence, the compressed regions and the spaced-out regions are referred to as compression and rarefaction respectively.
Answer:A. Compression and Rarefaction