All life must maintain an internal balance, despite environmental changes. this is called

stimulus
heredity
homeostasis
evolution

The answer is homeostasis because evolution is the gradual development of something, thus it cannot fit. Heredity can't fit because it deals with the genetic transmission of characteristics from a parent to a child and/or offspring. It also can not be stimulus because it refers to something that arouses or evokes action in something. Therefore, homeostasis is the only suitable answer as it deals with the tendency to acquire equilibrium within an internal environment.