How does fertilization occur in human reproduction?

Fertilization in human reproduction specifically occurs when a sperm cell penetrates an egg cell. This process marks the union of the male gamete (sperm) and the female gamete (egg), resulting in the formation of a zygote. The sperm undergoes several physiological changes as it approaches the egg, including the acrosome reaction, which allows it to penetrate the protective layers surrounding the egg. Once the sperm successfully enters the egg, their genetic materials combine to form a new organism with a complete set of chromosomes.

The other options do not accurately describe the process of fertilization. Fertilization does not involve the fertilization of a zygote, as a zygote is already a product of fertilization. Additionally, egg cell division occurs after fertilization, typically during the early stages of embryonic development, and is not part of the fertilization process itself. Furthermore, sperm reaching the ovary does not represent fertilization either, as fertilization specifically requires the sperm to meet and penetrate the egg, which occurs in the fallopian tubes, not the ovary.