Zygote is formed after the fertilization of male and female gametes. So zygote is the first cell that is produced after the fertilization step. Zygote contains DNA from male and female parents and the DNA divides as the zygote divides by mitosis.
So zygote is cell in which the DNA should be changed to provide the new DNA to every cell because the zygotic cell will divided and give rise to other cells in the organism. So the correct answer is 1.
