Which aquatic plant grows entirely below the water surface and may grow up to 6 feet long?

Elodea is the correct answer because it is an aquatic plant that thrives completely underwater and can reach lengths of up to 6 feet. It is a perennial plant commonly found in freshwater environments, where it grows in dense patches. Elodea is often used in aquariums and water gardens due to its ability to oxygenate water and provide habitat for aquatic life. Its submerged growth habit distinguishes it from other options, which either float on the water's surface or grow along the shore. For example, waterlilies have large floating leaves and flowers, cattails grow above the water with distinct upright stems, and purple loosestrife is a terrestrial plant that thrives in wetlands but is not aquatic. This unique characteristic of Elodea being fully submerged makes it suitable for specific ecological roles in aquatic ecosystems.