Answer: The correct answer is A) Fertilization.
Fertilization is a process or event of fusion of two gametes ( also called reproductive cells that are haploid) that forms zygote, which is diploid cell. It is the first event in the formation of new individual.
The two haploid gametes are sperm ( male gamete) and egg ( female gamete), which are formed through spermatogenesis and ovulation respectively.
Thus, fertilization is the process that joins two haploid cells to form a diploid zygote.