The correct option is A Gastrula
After zygote is formed, it divides by cleavage, which results in the formation of blastula. Later, gastrula is developed by the invagination of endodermal cells into ectodermal cells. The gastrula contains archenteron cavity, which opens through a wide aperture, called as the blastopore. The blastopore later becomes the mouth.
Thus, the correct answer is option (A).