Which process must occur first before any cell division can take place?

a. differentiation
b. embryo formation
c. blastocyst formation
d. fertilizatio

I would suggest you had put this in a different section. :D But I can answer it for you. 

I'm stuck between either B or D.

B. Embryo formation = a human or animal in the early stages of development before it is born, hatched, etc.

D. Fertilization = the joining of an egg cell and a sperm cell to form the first stage of an embryo

But technically the answer would be D. The act of creating the Embryo. 

Hope this helps. c:

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