Final answer:
The process by which offspring grow in size after the combination of egg and sperm is mitosis. Fertilization leads to a diploid zygote, and mitosis allows for the organism's development into an embryo and further into a fetus. The option (A) is correct.
Step-by-step explanation:
After human egg and sperm cells combine, the offspring grows in size through a process called mitosis. Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. When a sperm and egg merge at conception, they form a zygote, which begins as a single-cell structure. This zygote then undergoes multiple rounds of mitosis, which allows the organism to develop into an embryo and eventually grow into a fetus. The cell division involved in the growth of the offspring after conception is specifically through mitosis, where each division produces a genetically identical copy of the parent cell.
The event that leads to a diploid cell in a life cycle is fertilization. It involves the fusion of haploid gametes (an egg and sperm), forming a diploid zygote. This process ensures that the offspring has a complete set of chromosomes. After fertilization, the multicellular organism develops from the zygote by mitosis. Therefore, option (A) is correct.