What characteristic defines a filamentous fungus?

A filamentous fungus is characterized by its formation of branched filaments, known as hyphae. These hyphae combine to create a network called mycelium, which is essential for nutrient absorption and reproduction in the fungal organism. The branched nature of these filaments allows the fungus to spread efficiently through its substrate, facilitating better access to nutrients and moisture.

The other choices do not accurately represent filamentous fungi. Fungi are not single-celled; they are multicellular organisms, and thus do not possess a single, round cell structure. They also do not consist of small, independent organisms, as filamentous fungi thrive in a networked structure rather than individual components. Furthermore, filamentous fungi are not limited to dry environments; they can grow in various moisture levels, often thriving in moist habitats where they decompose organic matter.