Answer : The correct answer for the blank is -
a. zygote.
A zygote can be described as a diploid cell that is produced by the fusion of gametes (from each parent such as egg and sperm). The process of fusion is called fertilization and zygote is also called fertilized ovum/egg.
Zygote undergoes multiple rounds of proliferation (cell division through mitosis) and ultimately differentiates to produce the highly advanced stage, fetus (which is fully developed baby).
Thus, a zygote is formed when sperm meets egg and contains all DNA necessary to create a person.