Which of the following is NOT a type of invasive milfoil?

Hydrilla is not classified as a type of milfoil; instead, it is a different genus of aquatic plant known for its invasive properties. The confusion often arises because both hydrilla and milfoil can be problematic in aquatic ecosystems, but they belong to different families and exhibit different characteristics. Eurasian and variable milfoil are both types of milfoil; they share common traits related to their growth habits and their ability to disrupt aquatic environments. Parrot feather, although it is not a true milfoil, is still related to the group of invasive aquatic plants. This distinction makes hydrilla clearly the correct answer, as it stands apart from the milfoils mentioned in the options.