Dr. Skinner is a behaviorist. Your co-worker, Terry, asks the good doctor why Baby Herbie gives a happy yell, smacks his lips, and grins every time Terry opens the cookie jar. Dr. Skinner is most likely to answer that Herbie:

A is in the oral stage and is naturally stimulated by the presence of food.
B is in the primary circular reactions sensorimotor substage.
C has had his behavior rewarded with cookies in the past.
D is engaging in deferred imitation

My choice is B.