Which plant is a free-floating aquatic plant but is notably distinct for its size variability?

Duckweed is a free-floating aquatic plant known for its remarkable size variability. Species of duckweed can range considerably in size, with some being just a few millimeters across, while others can grow larger, even reaching a size comparable to that of a small dime. This variability allows duckweed to adapt to a variety of aquatic environments, and it often forms extensive mats on the surface of water bodies.

In contrast, other options present different characteristics. Elodea is a submerged aquatic plant rather than free-floating, making it unsuitable for this question. The American lotus, while it does have some floating aspects with its large leaves, is primarily distinguished by its sizeable, showy blooms and is not primarily classified as free-floating. Purple loosestrife, although an aquatic plant, is rooted in the substrate and does not exhibit the free-floating nature that defines duckweed. This highlights why duckweed is the correct choice for a free-floating aquatic plant known for distinct size variability.