The characteristics of a sound wave that we hear as pitch is the :

a. amplitude
b. velocity
c. spectrum
d. frequency

D.

Explanation:

Frequency can be defined as a number of waves that pass through a fixed place for a given amount of time. The pitch of sound is determined with the help of frequency. The unit of frequency is hertz(Hz). It is the speed of the vibration and is measured as a number of wave cycles per second.

For example, if the frequency is measured as 15 Hz, it means that there are 15 wave cycles per second.

So, the correct answer is option D.