Answer:
D. Homeostasis
Step-by-step explanation:
Homeostasis is the internal equilibrium maintained by the cells. If the homeostasis gets disturbed than diseases might occur. The cells maintain its equilibrium throughout the body. With the change in the external environment, the cells also adjusts itself to maintain its stability inside the body.
For example - at higher altitude, the oxygen concentration of the environment is less and to compensate the low oxygen content in the atmosphere, the number of red blood cell inside the body increases.