When a surfer rides an ocean wave on her surfboard, she is actually riding on
a. a crest that is toppling over.
b. a trough of the wave.
c. the rest position of the wave.
d. a region of rarefaction.

The correct answer is A. a crest that is toppling over.

This is also known as the top of the wave. Trough is the part where it's not a wave, like the bottom part that is at the sea level, between two waves. The period of time or space between two waves is called the wave length.