Which of the following belongs to the Basidiomycetes of importance?

The choice of Ustilago sp. as belonging to the Basidiomycetes is correct because Ustilago genera are classified within this group of fungi, which is characterized by forming spores on specialized structures called basidia. Ustilago species are often referred to as smuts, which are pathogens primarily affecting cereal crops, causing significant agricultural issues.

In contrast, the other options consist of fungi that belong to different groups. Aspergillus and Penicillium are classified as Ascomycetes, known for their ability to produce asexual spores called conidia and are commonly associated with food spoilage and certain diseases. Fusarium also belongs to the Ascomycetes and is critical due to its role in plant pathogens, causing wilt and rot diseases. None of these genera are part of the Basidiomycete classification, thus reinforcing the importance of Ustilago sp. within the context of plant pathogens.