Answer is D. maintaining structure.
When there is change in the external conditions, our body tends to maintain a constant internal environment so that every organ and cell can work to the optimum level. Maintenance of a constant internal conditions is called homeostasis. For examples, a healthy human body temperature should be 37°C. If the body temperature decreases below 37°C, metabolic reactions also decreases due slower movement of the molecules and have less kinetic energy. If the temperature increases above 37°C, the enzymes get denatured and metabolic reactions will be much slower.
