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

Elodea, also known as waterweed or anacharis, is an aquatic plant that grows entirely submerged in water and can reach lengths of up to 6 feet. This plant thrives in various aquatic environments and is characterized by its long, slender stems and flexible structure, which allows it to propagate and spread effectively in ponds, lakes, and slow-moving streams. The submerged growth habit of Elodea provides important benefits to aquatic ecosystems, such as serving as habitat for fish and invertebrates, contributing to oxygen production, and assisting in water clarity by absorbing nutrients.

In contrast, other plants listed in the choices have distinct growing habits or characteristics. Waterlilies have prominent floating leaves and flowers that emerge above the water surface. Cattails grow at the water's edge and feature tall, erect stems with flowering spikes, while purple loosestrife is a wetland plant known for its tall, flowering spikes that grow above the surface. Understanding the specific growth habits and environments of these plants is crucial for effective aquatic pest management.