The correct option is A Gastrula
The hollow spherical blastula is converted into a solid mass of cells the gastrula by a process which is known as gastrulation. In the gastrula the three germ layers can be seen, that is the ectoderm, endoderm and the mesoderm. The inner mass of the cell that is present in the blastula is responsible for the formation of the three germ layers. So, the correct option is 'Gastrula'.