Answer: a) drive-reduction
Explanation: Homeostasis is basically the body's need to maintain stability necessary for survival despite changes in the world outside. It is the body's unconscious tendency to maintain internal states (temperature, sleep, thirst, hunger) at stable levels.
Drive-reduction theory, which is a major theory of motivation, suggests that when there is an Homeostatic imbalance, needs are created, people perform actions (i.e their behaviour changes) to restore the body's ideal state, because of the need to restore balance.