Final answer:
The cell formed from the union of the nucleus of the ovum and the nucleus of the sperm during fertilization is known as a zygote.
Step-by-step explanation:
The student asks about the cell that is formed when the nucleus of the ovum and the nucleus of the sperm unite. This occurs during the fertilization process, where a sperm penetrates the protective layers around the oocyte, namely the corona radiata and zona pellucida, and subsequently fuses with the oocyte's plasma membrane. The haploid nucleus of the sperm is released into the cytoplasm of the oocyte, where it combines with the oocyte's haploid nucleus. This fusion culminates in the creation of a diploid cell known as a zygote, which contains the full set of genetic information necessary to develop into a new organism.
Therefore, the correct answer to the student's question is b. zygote.