Which process is essential for the development of multicellular organisms from a single fertilized egg?

The process that is essential for the development of multicellular organisms from a single fertilized egg is mitosis. After fertilization, when the sperm and egg combine to form a zygote, the zygote undergoes a series of cell divisions through mitosis. This process allows the single-celled zygote to divide repeatedly, leading to the formation of a multicellular organism.

Mitosis is crucial for growth, tissue repair, and the overall development of the organism as it ensures that each new cell receives an identical set of chromosomes as the original zygote. This genetic consistency is vital for maintaining the organism's identity and function as it develops from a single cell into a complex structure composed of many different cell types.

While fertilization is the initial step that combines genetic material, and implantation is critical for embedding the developing embryo into the uterine wall, neither of these processes contributes to the subsequent development of the organism at the cellular level in the way that mitosis does. Binary fission, on the other hand, is a method of asexual reproduction typically found in unicellular organisms, and does not apply to the development of multicellular organisms.