Select the Meaning of the Underlined Word in the Sentence

Select the meaning of the underlined word in the sentence.
Identical twins come from a single fertilized zygote that splits into two separate individuals.

A) an unborn offspring of a vertebrate animal that is still in the uterus or egg
B) an animal in the earliest stages of its development in the uterus or the egg
C) the cell that forms by the union of an egg cell and a sperm cell
D) an extremely young child or animal