The structure producing basidium in basidiomycetes is formed by the fusion of:
A. Two vegetative cells
B. Two male gametes
C. Two female gametes
D. Male and female gametes

Hint: Basidiomycetes are types of fungi. They are commonly known as the club fungi. Basidiomycetes are a type of fungi that produce sexual spores that are known as basidiospores.

Complete step by step answer:
They produce these on a club-shaped and spore-producing structure called the basidium. The basidium is the structure of the fungus that bears the basidiospores. They are found on the hymenophores of the fruiting bodies on the Basidiomycetes. The structure producing the basidium is formed by the fusion of two vegetative/ somatic cells of different strains or genotypes. Karyogamy (fusion of nuclei) and meiosis (reduction division) take place in basidium. The end product of this process is four basidiospores.
Hence, the correct answer is option A.
Additional Information:
Plasmogamy of the vegetative cells results in binucleate hyphae that are known as a dikaryon (one nucleus from the vegetative cells from each parent). Some of these cells undergo fusion of these two nuclei in the gills of the fruiting body. These now diploid cells are known as the basidia. The diploid and dikaryon phase is very brief. Soon after fusion, meiotic division takes place that results in the formation of four haploid nuclei (basidiospores). The other types of fungi ascomycetes, deuteromycetes, and phycomycetes. Basidiomycetes also reproduce asexually either by budding or asexual spore formation. Budding occurs when a vegetative outgrowth of the parent cell is separated from the parent body into new cells. Any cell in this organism can form buds. Asexual spore formation takes place at the ends of specialized structures known as conidiophores.

Note: A hymenophore is a hymenium (the surface that bears the spores) bearing the structure of a fungal fruiting body. They can have smooth surfaces and be lamellae, folds, tubes, or teeth shaped
Mushrooms (Agaricus), puffballs, stinkhorns, rusts, and smuts are the five main divisions of basidiomycetes.