The correct option is A Karyogamy
Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two nuclei. In order for karyogamy to occur, the cell membrane and cytoplasm of each cell must fuse with the other in a process, known as plasmogamy. Once within the joined cell membrane, the nuclei are referred to as pronuclei. Once the cell membranes, cytoplasm, and pronuclei fuse together, the resulting single cell is dipliod, containing two copies of the genome. This diploid cell, called as a zygote can then enter meiosis. Mammalian fertilization uses this process to combine haploid sperm and egg cells to create a diploid fertilized egg.