The correct option is D Waterfall
Fast-flow of river in the mountainous region causes formation of rapids. Shallow, fast-flowing water in a stream is called a rapid. Waterfalls often flow from soft rock to hard rock. This happens both laterally (as a stream flows across the earth) and vertically (as the stream drops in a waterfall). In both cases, the soft rock erodes, leaving a hard ledge over which the stream falls.