An ascus is to ascomycetes as is a _ to basidiomycetes:

Explanation

The correct answer is Basidium. In ascomycetes, an ascus is the structure in which spores, known as ascospores, are produced. Similarly, in basidiomycetes, the basidium is the spore-bearing structure where basidiospores are produced. A basidiospore is a type of spore produced by the basidium, not the structure itself, thus it is incorrect. A basidiocarp is a fruiting body that contains basidia but is not analogous to an ascus. Finally, a haustorium is unrelated to the reproductive structures in these fungi.