The correct answer is b) The organism undergoes both cellular division and differentiation.
After a zygote is formed through the fertilization of an egg and sperm, it begins to divide through a process called mitosis. As these divisions continue, the cells also undergo differentiation, which means they start to develop into specialized cell types that will form different tissues and organs within the multicellular organism. This combination of cellular division and differentiation is essential for the development of a complex organism from a single cell.