Which natural feature is described as one of the largest in the world?

The correct choice is 'cascade.' Cascades can refer to natural formations where water flows over a vertical drop, which can be quite large and extensive in certain geographical locations. Some cascades can be considered among the largest in the world due to their height, volume, and the unique geological formations that accompany them.

While waterfalls are also large water features, "cascade" can encompass a broader range of multi-tiered waterfalls or large areas where water flows over rocks, thereby being recognized as significant natural formations. Rivers, while they can be massive and extensive, do not emphasize the verticality or the multi-step structure often associated with cascades. Mountains, on the other hand, are significant natural features, but they do not fall under the same category as cascades, which are specifically related to flowing water.

In the case of this question, recognizing the nuances of these natural features, particularly how cascades can represent some of the most dramatic flowing water formations, helps justify why 'cascade' is the most fitting answer.