Jeff's Parents Want Him to Study More

Jeff's parents want him to study more. To do this, they tell him that if he studies two hours per night during the week, they will pay for him to go to the movies with a friend on the weekend. After a month, they notice that Jeff is studying more often and more
Consistently. According to the principles of operant conditioning, getting money to go to the movies would be a(n) :

A) reinforcer.
B) discriminative stimulus.
C) unconditioned stimulus.
D) punishment.