Ans. Mitosis.
Mitosis is a process of cell division, which involves formation of two daughter cells from a parent cell having same genetic constitution as present in parent cell. It plays role in growth, development, and repairing of damage tissue.
After fertilization, a diploid, single-celled zygote is formed, which goes for mitosis, make thousands of cells and develops into fetus, which later develops into an adult animal.
Thus, the mechanism by which a diploid animal grows after fertilization is 'mitosis.'