A boy with autism sometimes engages in self-stimulatory behavior such as waving his hands in front of his eyes while doing his homework. Curiously, he engages in this behavior when his father is around but usually not when his mother is around. An applied behavior analyst working with the boy closely observes the interactions between the boy and his parents in an effort to determine why the self-stimulatory behavior is occurring. She notes that, on the rare occasions when the boy engages in the problem behavior in the presence of his mother, the mother simply ignores this behavior. However, after the problem behavior occurs a number of times in the presence of the father, the father often intervenes. While he does not provide attention to his son, he does remove the homework materials for a number of minutes, essentially giving the boy a break. This usually calms the boy and results in the cessation of the problem behavior. The therapist notes that the amount of self-stimulatory behavior before the father intervenes is quite unpredictable; sometimes the father intervenes after only one or two instances, and sometimes he waits until many instances occur in an effort to keep the boy on task with his homework as long as possible. The boy's self-stimulatory behavior is an example of:

a. behavior maintained by positive reinforcement.
b. observational learning.
c. behavior maintained by negative reinforcement.
d. a classically conditioned response triggered by the father