Which of the following terms is used to describe what causes organisms to react to their environment?

a. stimulus
b. camouflage
c. parasites
d. homeostasis

homeostasis.

Homeostasis is the regulation of the functions of some internal conditions as reaction to changes in the environment.

For example, although the external temparuture changes the matabolism is capable to keep the temperauture of the body constants through homeostasis.

That does not happen with the cold blooded animals. If you put a small frog very close to a hot ligthed bulb, its blood will not be able to keep it cool and it will  dry completely.