Explanation:

A crest is the highest point the medium rises to and a trough is the lowest point the medium sinks to. ... A crest is a point on the wave where the displacement of the medium is at a maximum. A point on the wave is a trough if the displacement of the medium at that point is at a minimum.
Peaks means the highest point. Generally, peaks have their own names, which differ from the name of the mountain they're part of. So mountain is broader term and peak is narrower one. A mountain can have many peaks.