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

The choice of Duckweed as the correct answer highlights its unique characteristics among free-floating aquatic plants. Duckweed is known for its small, diverse size, varying from just a few millimeters to a couple of centimeters in length. This variability allows it to adapt to different environmental conditions, making it a crucial component of many aquatic ecosystems. Duckweed can rapidly reproduce and form dense mats on water surfaces, providing habitats for various aquatic organisms and influencing water quality by absorbing excess nutrients.

In contrast, the other plants listed have distinct growth forms and characteristics that set them apart. Elodea is a submerged aquatic plant and does not float freely on the surface like Duckweed. The American lotus is characterized by its large, broad leaves that float on the water’s surface and a prominent flowering structure visible above the water, making it significantly larger than Duckweed. Purple loosestrife, while an emergent plant, is known for its tall, upright growth and can dominate wetland areas, but it does not exhibit the free-floating property that Duckweed possesses. This distinction underlines the reason why Duckweed is recognized for its size variability among free-floating aquatic plants.