The correct answer is: D. Cell division.
A multicellular organism grows by cell division. This is the process by which a cell divides into two daughter cells. The daughter cells are genetically identical to the parent cell. Cell division is essential for the growth and development of multicellular organisms. It also allows for the repair of damaged cells and the production of new cells to replace old ones.
Option A, cell addition, is incorrect because it does not describe the process of cell division. Cell addition is the process by which cells are added to a structure. This can happen through the growth of existing cells or the addition of new cells.
Option B, cell explosion, is incorrect because it does not describe a natural process. Cell explosion is a type of cell death that occurs when a cell is damaged beyond repair.
Option C, cell implosion, is incorrect because it does not describe a natural process. Cell implosion is a type of cell death that occurs when a cell is deprived of nutrients.