Final answer:
Gastrulation is the process that rearranges the cells of a blastula into a gastrula, characterized by three germ layers which give rise to different organ systems.
Step-by-step explanation:
The process of gastrulation forms the gastrula. The stages of embryonic development begin with fertilization, followed by cleavage, during which the single-celled zygote undergoes rapid cell division. The resulting structure, the blastula, then undergoes gastrulation where the cells rearrange spatially and fold in on themselves to form three germ layers: the ectoderm, mesoderm, and endoderm. These layers eventually differentiate into different organ systems, making the gastrula the correct answer.