The answer should be homeostasis.
Examples of homeoststais in human, the ability to maintain a constant internal environment includes maintaining body temperature, where we would shiver when cold so that our muscles can move and respire so heat is generated, or we sweat when hot, so that our body can cool down. Another example includes maintaining a constant blood glucose concentration, if our blood glucose is too high, insulin (a hormone) is secreted, or if it's too low, another hormone called glucagon is secreted.
On the other hand, all of the other choices are not the right answer. Stimulus response is where we respond to certain stimulus, where nerve cells are involved. For example, we pull our hand away if we touch hot stuff. Respiration is just the exchange of oxygen and glucose to carbon dioxide and water to produce energy. It is how the cells generate energy. And metabolism is just generally the chemical reactions within our body.
Therefore your answer is homeostasis.
