What type of plant relies on surrounding water for support and grows entirely underwater?

Submersed plants are specifically adapted to live entirely underwater. They rely on the surrounding water for structural support, as their tissues are designed to thrive in a submerged environment. These plants typically possess thin, flexible stems and leaves that can withstand the pressure and movement of the water. They play a crucial role in aquatic ecosystems by providing habitat for aquatic organisms, improving water quality through oxygen production, and stabilizing sediments.

Other plant types, such as emersed plants, are rooted in the substrate but grow above water, while floating leaf plants partially float on the water's surface. Aerial plants are entirely above water and are not dependent on aquatic environments for their growth and support. Thus, the defining characteristic of submersed plants—being entirely underwater—is what makes this choice the correct answer.