Answer: Trough
Explanation: The point labeled C in the wave diagram above is the TROUGH of the wave motion. The trough of a wave motion identifies or signifies the point of least or minimum Displacement by measuring the downward Displacement of the wave. The point A is the CREST which is the opposite of the trough, signifying the point of maximum or upward Displacement of the wave cycle.
Point B is the wave amplitude which signifies the maximum extent of vibration from the equilibrium position of a wave. The point labeled D refers to the wavength of the wave motion which is the distance between successive crest or troughs of a wave motion.