What type of plant fully grows beneath the water surface and relies on water for support?

The correct answer is submersed plants. These types of aquatic plants are specifically adapted to grow entirely underwater. They develop various structures to thrive in a submerged environment, such as flexible stems and leaves that can withstand the currents and pressure of water. These plants not only rely on water for support but also obtain nutrients and perform photosynthesis while fully submerged.

Emergent plants, on the other hand, grow partially above the water surface, which distinguishes them from submersed plants. Floating plants are those that float on the water's surface and may have some portions submerged but do not grow entirely underwater. Terrestrial plants, meanwhile, grow on land and are not adapted to aquatic conditions at all. Thus, submersed plants represent a unique category in aquatic ecosystems, crucial for habitat formation and water quality improvement.