Which of the following plants has deeply serrated leaf margins and grows up to 6 feet long?

Eurasian watermilfoil is recognized for its deeply serrated leaf margins, which is a key characteristic that helps in its identification. The leaves of this aquatic plant typically have a finely dissected structure, resembling feathers, and can extend up to 6 feet in length. This distinctive morphology enables it to thrive in a variety of aquatic environments, often leading to dense mats that can disrupt local ecosystems.

In contrast, Southern naiad, Curly-leaf pondweed, and Brittle naiad possess different leaf structures and characteristics. For instance, Southern naiad's leaves are generally more lanceolate and do not have the same serrated margin. Curly-leaf pondweed exhibits a distinctive curling of its leaves but lacks the deeply serrated edges typical of Eurasian watermilfoil. Lastly, Brittle naiad features long, narrow leaves and is typically less robust in overall structure compared to Eurasian watermilfoil. Thus, the specific characteristics of the leaf margins and growth habits uniquely identify Eurasian watermilfoil as the correct answer.