Answer: Fertilization
Fertilization is the process in which there is union of female egg and male sperm, which takes place at the ampullary-isthmic junction of the Fallopian tube and it ultimately results in the formation of zygote.
During the process of sexual reproduction, when there occurs fertilization of ovum (n, 23 chromosomes) by the sperm (n, 23 chromosomes) in order to restore the diploid number and creates a zygote (2n, 46 chromosomes) that divides and grow by the process of mitosis in order to form a multi-cellular human.