What is the name of the aquatic plant type that is not attached to sediment and floats on or just below the water surface?

The term for the aquatic plant type that is not anchored to the sediment and floats on or just below the water surface is indeed referred to as free-floating. Free-floating plants are characterized by their ability to float freely in the water, deriving nutrients from the water itself rather than through roots that reach into the sediment. They can often be seen on the surface of bodies of water, and examples include species like duckweed and water hyacinth.

In contrast, submersed plants are fully underwater and anchored to the sediment, while rooted-floating plants are attached to the sediment but have leaves and stems that float above the water's surface. Emergent plants, on the other hand, are rooted in the sediment and grow above the waterline, making them distinctly different from free-floating plants. Thus, the identification of free-floating as the correct answer highlights its specific characteristics and ecological role in aquatic environments.