Answer:
The correct answer is option A, Gastrula
Step-by-step explanation:
Gastrula has three layers of cell and it is the phase in which single layered blastula matures and get converted into multilayered structure.
However, Morula is formed after the formation of blastula through the process of blastulation. In the Morula phase, blastomers get arranged in a spherical shape after the zygote split. And zygote is formed at a very early stage of fertilization .
Thus , gastrula is the correct answer as it is the stage which comes after zygote and morula and thus get developed into more compact mass of cells.