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

Eurasian watermilfoil is characterized by its finely dissected, feathery leaves that have deeply serrated margins. These leaves can grow in dense whorls and typically reach a length of up to 6 feet. This aquatic plant is commonly found in freshwater systems and is known for its rapid growth and ability to form dense mats on the water surface, which can inhibit light penetration and disrupt local ecosystems.

The other options are broader-leaved plants or do not exhibit the same level of serration in their leaf margins. Southern naiad, for instance, has a different leaf structure and grows differently in comparison to Eurasian watermilfoil. Curly-leaf pondweed does have somewhat serrated leaves but does not reach the same height or have the same characteristics as Eurasian watermilfoil. Similarly, brittle naiad features leaves that are not deeply serrated and has distinctive growth forms that set it apart.

Thus, based on its distinct leaf characteristics and growth attributes, it is clear that Eurasian watermilfoil is the correct choice for this description.