Final answer:
The actual moment of fertilization occurs as maternal and paternal chromosomes combine to form the diploid zygote, when the genetic material from the pronuclei intermingles after their replication and migration towards each other.
Step-by-step explanation:
The actual moment of fertilization occurs as maternal and paternal chromosomes combine and produce the diploid zygote. This happens when the two haploid nuclei, which are derived from the sperm and the oocyte (also known as the ovum once meiosis is complete), referred to as pronuclei, decondense, expand, and replicate their DNA in preparation for mitosis. Following replication, the pronuclei migrate toward each other, their nuclear envelopes disintegrate, and the genetic material intermingles, resulting in the formation of a single-celled diploid zygote. This step concludes the process of fertilization, equipping the zygote with all the genetic instructions necessary to develop into a human. Hence, the correct answer to the question is 'A) Maternal and paternal chromosomes combine and produce the diploid zygote'.