Answer: (d)
Homeostatic control includes many variables, such as body temperature and fluid or moisture level, being kept within certain pre-set limits (homeostatic range). Other variables include the pH or acidity level of extracellular fluid, the concentrations of sodium, potassium and calcium ions, as well as that of the blood sugar level, and these need to be regulated despite changes in the environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life.
The ability of an organism or organ to respond to external stimuli is called sensitivity. It is not under homeostatic control.