The amplitude of a wave is

The amplitude of a wave is a measure of the displacement of the wave from its rest position. The amplitude is shown on the graph below. Amplitude is generally calculated by looking on a graph of a wave and measuring the height of the wave from the resting position. The amplitude is a measure of the strength or intensity of the wave.

Explanation:

Hope this helps! <3

i am pretty sure it is c because when people are closer together, they cannot move as freely. picture being in a mostly full elevator, for instance. the people are right next to you, and you can barely move.