What type of plant grows entirely beneath the water surface?

Submergent plants are specifically adapted to grow entirely underwater, which is what distinguishes them from other types of aquatic vegetation. These plants, such as eelgrass and various types of pondweeds, have specialized structures that allow them to photosynthesize efficiently and take in nutrients while being fully submerged. Their leaves are often slender and flexible, which helps them cope with water currents and the pressure of the water above.

Floating plants, in contrast, are those that remain on the surface of the water, relying on buoyancy to hold them up. Emergent plants grow from the sediment at the bottom of water bodies but have parts that extend above the water surface, making them partially submerged. Ferns, while they can be found in moist environments, do not primarily grow underwater and are classified differently in the plant kingdom. Overall, the defining feature of submergent plants is their complete underwater growth, making them the correct choice.