When your professor arrives and opens his notes, you stop talking to the person next to you and prepare to take notes. This situation is most clearly an example of

A) classical conditioning.
B) belongingness.
C) a discriminative stimulus.
D) an unconditioned reinforcer.

A discriminative stimulus is a cue or signal that indicates the availability of reinforcement for a particular behavior. In this case, the professor's arrival and opening of notes signals that it is time to take notes, which is reinforced by successful learning and potentially good grades. The behavior of stopping to talk and preparing to take notes is being reinforced by the context and cues of the situation, making it a discriminative stimulus. Classical conditioning, belongingness, and unconditioned reinforcers don't align as well with the scenario described.