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

The term that specifically refers to the downward movement of water in a waterfall is "plunge." In the context of waterfalls, a plunge occurs when water flows over an edge, falling vertically and creating a distinct drop into the pool or river below. This aspect of a waterfall is characterized by its dramatic descent, which is a key feature often highlighted in discussions of hydrology and erosional processes.

Other options, while related to water movement, do not accurately describe the vertical descent in the same way. "Drop" can refer to any downward motion but does not specifically capture the essence of a waterfall's plunge. "Cascade" implies a series of small waterfalls or a flow over rocks, suggesting a gentler descent rather than the dramatic drop of a plunge. "Flow," in general, refers to the movement of water and lacks the specificity of the vertical descent that defines a plunge.