Answer:
d. Wave Height
Explanation:
Imagine my crude drawing of a sea wave is actually passable haha, so, the top of the wave is called the crest, and the lowest part of the wave, under the calm sea level, is the Trough, the vertical distance between those two points is the wave height.
As extra credit ;) the distance between two consecutive crests or two consecutive troughs is the wavelength, and this is exactly the same in radio waves!
