The surface area/volume ratio is an important factor for one celled organisms. as a cell's volume grows, its surface area/volume ratio decreases, making it harder and harder to acquire enough nutrients via diffusion. because of this, cells stop growing after reaching a certain size. after reaching this state, a one celled organism is likely to

Answer

Use Mitosis to divide and create daughter cells.

Explanations  

Single-celled organisms use mitosis to reproduce. Both growth and reproduction in unicellular organisms are mutually inclusive. When the cell’s volume grows, the ratio of surface area to volume decreases creating challenges in acquisition of nutrients because there will be too much cytoplasm for a given amount of nuclear material , thus the cell divides by mitosis to reproduce daughter cells and the process begins again.

the correct answer is:

a population changes in response to a change in the environment.