What phenomenon occurs when male and female nuclei fuse in cellular reproduction?

The correct choice is karyogamy, which refers to the fusion of male and female nuclei during cellular reproduction. This process is crucial for sexual reproduction in many organisms, particularly in fungi and certain protists. During karyogamy, the genetic material from two parent cells combines, resulting in a diploid nucleus. This step is essential to restore the diploid state after meiosis, which produces haploid gametes.

Mitosis involves the replication and distribution of a cell's genetic material into two daughter cells but does not involve the fusion of two distinct nuclei. Meiosis is a type of cell division that reduces the chromosome number by half, resulting in haploid gametes, but does not include the merging of parental nuclei—that step is later, during fertilization, in many life cycles. Plasmogamy is the stage where the cytoplasm of two compatible mating types merges, but it does not involve the fusion of their nuclei; that occurs afterward in karyogamy, completing the fertilization cycle.