The correct option is:
crest trough
In fact, the sentence can be completed as follows:
"The amplitude of a wave is the height of a wave as measured from the highest point on the wave crest to the lowest point on the wave trough."
Explanation:
The amplitude of a wave is equal to the height of a wave. It is the maximum displacement of the wave. The intensity of a wave is directly proportional to the square of the amplitude of a wave.
The highest point of the wave is called the crest and the lowest point of the wave is called the trough.
The crest of wave is defined as the distance from the base line to the maximum point and the trough of a wave is defined as the distance from the base line to the minimum point.
