Answer: Zygote.
Explanation: The differentiation takes place at the stage of zygote. Cell differentiation is the process of development during which the embryonic cell get specialized and diverse structure arises.
Once an egg is fertilized and develops into a zygote, it further divides into multiple cells, triggering the process of cell differentiation.
Every cell of the body is equipped for a special process. The differentiation of cell to form tissue and then differentiation in tissues to form organ and then finally into a whole organism.