Final answer:
In Ascomycetes and Basidiomycetes, the short time gap occurs between a) plasmogamy (fusion of cells) and karyogamy (fusion of nuclei), with the former establishing a di-karyotic cell that later undergoes nuclear fusion in karyogamy.
Step-by-step explanation:
The short time gap in the life cycle of Ascomycetes and Basidiomycetes is observed between plasmogamy and karyogamy. During the sexual reproduction of these fungi, plasmogamy is the first step in which the fusion of cells takes place, resulting in a di-karyotic cell that contains two haploid nuclei from each parent.
This cellular union forms a cell with two nuclei, each retaining its own identity, hence the cell is not considered diploid but rather referred to as di-karyotic. This di-karyotic state can be maintained as the cell divides and grows. Eventually, karyogamy occurs, where these two haploid nuclei fuse to form a diploid nucleus. This is followed relatively quickly by meiosis, which restores the haploid state in preparation for spore production.
Hence, the correct answer is Option A.