It is crucial that the process or mitosis is error free. this is because otherwise healthy cells can

The answer is become cancer cells. 

Mitosis is the process of cell reproduction. A cell cannot live forever so they either divide on two new cells through mitosis or die. If something happens to the control of mitosis, the healthy cells will divide uncontrollably and these new cells, cancer cells, will also be out of control. These cancer cells will not die, but they will continue dividing and will form tumors at one moment.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.

i think the answer is b.