Which of the following best characterizes a filamentous fungus?

A filamentous fungus is best characterized by its multicellular structure and branched arrangement of cells. These fungi develop a network of thread-like structures called hyphae, which collectively form mycelium. This complex arrangement allows filamentous fungi to effectively absorb nutrients and adapt to various environments.

The characteristic of being multicellular and having a branched cellular arrangement distinguishes them from other forms of fungi and microorganisms. Simple single-cell organisms, such as yeasts, do not exhibit this branching structure. While some filamentous fungi may have high moisture requirements, this is not the defining characteristic of their structure and growth form, which is central to understanding types of fungi. Therefore, the multicellularity and branching are the key features that define filamentous fungi, making this the best answer.