Answer;
C. cell differentiation has begun.
Step-by-step explanation;
Blastula is a hollow sphere of cells, or blastomeres, produced during the development of an embryo by repeated cleavage of a fertilized egg.
Cellular differentiation is the process where by a less specialized cell becomes a more specialized cell type.
The cells In the blastula stage are not differentiated, its a mass of cells which has not undergone differentiation and specialization to undertake specific roles. Cell differentiation in the embryonic development starts at the embryo stage of the embryonic development.
Differentiation of cells in the embryo is as a result of both internal cellular factors and extracellular factors that act on the cell from the outside.