Which of the following terms refers to the number of compressions or rarefactions per unit of time?

The number of compressions or rarefactions per unit of time is called the frequency of the sound wave.
Frequency is determined as the number of crests or troughs per unit of time. In other words, it can be defined as the number of compressions or rarefactions per unit of time. When the vibrating object pushes the air particles, they create compression. In contrast, when the vibrating object moves back, it creates a gap, i.e., rarefaction.
 

Hence, the correct option is frequency.