homework - Is Aspergillus clavatus an unicellular organism?

You're right in saying that yeast is single celled.

However, moulds are described to be filamentous fungi that are multicellular. The filaments of the mould give colonies "a woolly, fluffy, or velvety appearance, sometimes punctuated with a granular or powdery aspect that is produced by the formation of asexual reproductive structures"(1).

Aspergillus is such a species. The filaments are called hyphae. In Aspergillus, hyphae have divisions or walls which separate it into multiple cells. This is called septate hyphae.

1. McPherson, et al. Henry's Clinical Diagnosis and Management by Laboratory Methods. 22nd ed. 2011. Saunders.