There are two parts to a wave, the

a) crest and the trough

b) wavelength and base

c) frequency and speed

d) crest and color

Start with the process of elimination. Obviously, color is not a specific component to a wave, so we can cancel that out. While there are frequencies and measurable speeds of waves, those are not considered parts of the wave; nor is the wavelength or the base considered to be a part of the wave. The crest is the highest point of the wave, and is considered a part of the wave, as well as the trough, which is the the lowest point on the wave.

The answer options is d