At what stage does a fertilized egg begin to undergo cell division?

The correct choice is identified as the process known as cleavage. After fertilization occurs, the fertilized egg, or zygote, begins to divide through a series of mitotic divisions called cleavage. This process starts within the first few days after fertilization, where the single cell divides multiple times without the overall growth of the embryo, leading to the formation of a multicellular structure called the morula.

Fertilization refers specifically to the initial fusion of the sperm and egg, while gastrulation is a later developmental phase that involves the reorganization of cells into germ layers. Implantation occurs when the blastocyst attaches to the uterus lining, a step that takes place after cleavage has already begun. Therefore, cleavage is the correct answer as it specifically denotes the initial cell divisions following fertilization.