Which plant is commonly referred to as "pond scum" and is recognized by its greenish brown mats formed on the water's surface?

The term "pond scum" primarily refers to filamentous algae, which are often found in freshwater environments. These algae can proliferate quickly and form dense, greenish brown mats that float on the surface of the water, giving them their colloquial name. This growth can obstruct sunlight from reaching other aquatic plants and can impact dissolved oxygen levels in the water, affecting fish and other aquatic organisms.

In contrast, the other plants listed, such as sago pondweed, curly-leaf pondweed, and wild celery, do not typically form these distinct surface mats. Sago pondweed and wild celery are rooted aquatic plants that grow underwater and are beneficial to aquatic ecosystems. Curly-leaf pondweed does have a different growth form, appearing as a submerged plant rather than forming surface mats, and it has unique curly leaves. Therefore, filamentous algae is the accurate choice for identifying "pond scum" based on its physical characteristics and ecological impact.