Which of the following is a free-floating hydrophyte?

Lemna, commonly known as duckweed, is a free-floating hydrophyte, which means it thrives in aquatic environments without anchoring to the substrate. Duckweeds are small, green, and often form dense colonies on the surface of ponds and slow-moving waters, where they utilize sunlight for photosynthesis. Their floating nature allows them to efficiently access nutrients from the water and facilitates rapid growth and reproduction.

The other listed plants have different adaptations; for instance, Isoetes, or quillwort, typically grows in submerged conditions but is rooted in the substrate. Vallisneria, also a submerged aquatic plant, has a significant anchorage in sediment and can grow tall, while Potamogeton represents a genus of aquatic plants that are predominantly rooted in the water, with some species exhibiting floating leaves but not free-floating characteristics like duckweed. Hence, Lemna is definitively classified as a free-floating hydrophyte, distinguishing it from the others based on its growth habits and ecological adaptations.