While Out Trick-Or-Treating for Halloween,at First Billy's Neighbours Had to Remind

Multiple Choice

While out trick-or-treating for Halloween,at first Billy's neighbours had to remind him to say "trick or treat" before they dropped their treats into his bag.As he continued to visit houses in the neighbourhood,soon Billy would say "trick or treat" as soon as the neighbours answered the door.In this situation,which operant conditioning term describes the treats he received for the behaviour of saying "trick or treat"

A)  reward
B)  discriminative stimulus 
C)  conditioned stimulus
D)  reinforcement