The correct answer is:
c) Normal cells revert to a more fetal appearance and function, probably because of mutation of cellular genes called proto-oncogenes.
When a tumor is described as poorly differentiated, it means that the cells within the tumor have lost some of their normal characteristics and resemble less mature or more primitive cells. This is often a result of mutations in genes, including proto-oncogenes, which can lead to uncontrolled cell growth and reduced cellular differentiation.