Cambridge Secondary (7-9). Stage 9. Science. Learner's Book - Unit 6: Sound and space

Question 6

Which of these describes the frequency of a sound wave? Write one letter.

The maximum distance that particles move from their position when there is no wave.
The total distance that particles move forward and move backward in the wave.
The number of times that particles vibrate backwards and forwards every second.
The direction that particles move in the wave compared to when there is no wave.