Answer
B.
Work Step by Step
If it were not for cytokinesis, cells would double the number of their nuclei at each mitosis, causing chaotic, gigantic cells to form, so B. is correct. A. is referring to mitotic separation of chromosomes, not cytokinesis, and C. and D. refer to the cell cycle in a more general way, not the specific purpose of cytokinesis, which is what the questions is asking about.