Which type of vascular aquatic plant is either free floating or anchored and produces most of their leaf stem tissue at or above the water surface?

The correct choice is based on the characteristics of aquatic plants. Floating vascular aquatic plants are defined by their ability to thrive on the water's surface, exhibiting a unique adaptation that allows a significant portion of their leaf and stem tissue to remain above the water. This adaptation aids in maximizing sunlight exposure for photosynthesis and improving gas exchange.

Emergent plants also grow in water but are typically rooted in the substrate with stems and leaves that extend above the water's surface. In contrast, floating plants do not require anchoring to the bottom and can freely float on the surface, distinguishing them from emergent species.

Submersed plants grow entirely underwater and therefore do not produce significant above-water tissue, while planktonic refers to microscopic organisms that drift in the water column and do not have the structural features of vascular plants. Hence, the defining characteristics of floating plants align with the description in the question, making it the appropriate selection.