Which aquatic plant type can be found both free-floating and anchored in the substrate?

The correct choice identifies floating plants as those that can both float freely on the surface of the water and have the capability to anchor themselves in the substrate under certain conditions. Free-floating aquatic plants, such as water hyacinth or duckweed, primarily exist on the water's surface without rooting in the soil. However, some species possess adaptations that allow them to establish themselves in the substrate when conditions are suitable, thereby leading to anchored growth.

In contrast, emersed plants are typically rooted in the substrate and grow above the water's surface, submersed plants fully inhabit the water column and are anchored to the substrate, and vascular plants, while they include both types, do not specifically denote the characteristic of being free-floating and anchored simultaneously. Thus, floating plants represent the unique ability to occupy both ecological niches effectively, affirming their classification in this context.