Loudness is to amplitude as pitch is to

A) intensity.
B) decibels.
C) rhythm.
D) frequency.

Amplitude

The extent of a vibratory movement, measured from the mean position to an extreme, commonly used in physics to describe waves.

Pitch

The quality of sound governed by the rate of vibrations producing it; the highness or lowness of a tone.

Frequency

In the context of waves, it is the number of waves that pass a fixed point per second; in general use, it refers to how often something occurs.