Which aquatic plant is characterized by visible coarse teeth on leaf margins?

The aquatic plant that is characterized by visible coarse teeth on leaf margins is Hydrilla. This plant features long, thin leaves that grow in whorls around the stem, and each leaf exhibits noticeable serrations or coarse teeth along the edges. These distinctive leaf margins are a key identifying feature of Hydrilla, making it easier to distinguish from other aquatic plants.

Water shield is typically recognized by its rounded, shield-like leaves and fleshy stems, lacking the serrated edges found in Hydrilla. American frogbit, on the other hand, has rounded leaves that float on the water's surface and do not present any coarse teeth on the margins. Variable leaf milfoil features finely dissected leaves that resemble feathers and also lacks the coarse teeth, contributing to its unique identification away from Hydrilla. Thus, Hydrilla's noticeable toothy leaf margins set it apart as the correct choice in this context.