The correct option is C A series of compressions and rarefactions
During vibration, when a particle moves forward, it compresses the air in front of it and creates a high pressure or high density region. This region is known as compression.
When a particle moves backward, it creates a low pressure or low density region. This region is known as rarefaction.
When sound propagates in a medium, the particles of the medium vibrate along the direction of propagation of wave.
Therefore, sound wave is a longitudinal wave.