Final answer:
The formation of gametes (d) is not a reason why cells undergo mitosis, as this is achieved through meiosis, which creates haploid cells for sexual reproduction.
Step-by-step explanation:
The option that is not a reason why cells undergo mitosis is 'd) formation of gametes'. Mitosis is the process by which a cell divides to produce two new daughter cells that are genetically identical to the original cell. This process fulfills several purposes including repair of organs and tissues, regeneration of cells, and contributing to growth by increasing the number of cells. It is also involved in asexual reproduction in some organisms.
Conversely, the formation of gametes, which are the sex cells used in sexual reproduction, is achieved through a different process called meiosis. Meiosis is a two-phase cell division that reduces the chromosome number by half, resulting in four haploid cells that are genetically distinct from the original diploid cell and from each other — which is essential for genetic diversity in sexually reproducing organisms.