Which aquatic plant has flattened, winged, red-brown brittle stems that grow up to 6 feet long?

The correct choice is curly-leaf pondweed. This aquatic plant is characterized by its elongated stems that can grow to lengths of up to 6 feet and are distinctly flattened and winged. Its stems are also known for their brittle nature, making it easily distinguishable from other aquatic plants.

Curly-leaf pondweed thrives in various water bodies and is noted for its often reddish-brown coloration, especially during certain growth stages. The morphology of its stems, with the combination of being flattened and having a winged structure, provides a unique identification feature that aligns precisely with the description provided in the question.

In contrast, other options such as clasping-leaf pondweed, brittle naiad, and southern naiad do not possess the same stem characteristics and thus do not match the description given, making them less suitable answers. Understanding these specific traits of curly-leaf pondweed is important in aquatic pest management, particularly in identifying and distinguishing between different aquatic plants in various environments.