Which of the following refers to the downward movement of water in a waterfall?

The term that refers to the downward movement of water in a waterfall is "plunge." This specifically describes the way water falls from a height, giving waterfalls their distinctive and dramatic characteristic. When water cascades over a ledge or cliff, it often free-falls, creating a plunge pool at the base due to the impact of the water.

The concept of a "drop" could seem related, but it is more general and does not specifically highlight the dynamic nature of the water as it moves down vertically. "Cascade" typically means a series of small waterfalls or the manner in which water flows over a steep surface, which differs from the singular, vertical drop of a waterfall. "Flow" describes water moving in a more general sense, not specifically relating to the abrupt or vertical movement found in waterfalls. Thus, "plunge" distinctly captures the essence of how water behaves in this context.