A plant that grows completely below the water surface and relies on the surrounding water for support is best described as ____________.

The correct description of a plant that grows completely below the water surface and relies on the surrounding water for support is "submerged." Submerged plants typically have their entire structure beneath the surface of the water, which allows them to absorb nutrients directly from the water and utilize photosynthesis while being fully supported by the aquatic environment. They can play a critical role in maintaining water quality and providing habitat for aquatic organisms.

Emergent plants are those that grow partly above the water surface, with their roots often anchored in the substrate below. Floating plants remain primarily on the water's surface, with parts of the plant above water while their roots may dangle freely in the water. Filamentous plants, on the other hand, usually refer to algae or plant forms that grow in thin thread-like shapes.

Understanding these distinctions is essential for effective aquatic weed management and identifying the ecological roles various plants play in aquatic ecosystems.