Apigeon learns to peck at a disk lighted green to receive reinforcement but not at a disk lighted red. this means that, for the pigeon, the color of the disk is a:

discriminative stimulus.

Explanation:

In operant conditioning, we call the stimulus that points to the subject the availability of reinforcement discriminative. For example, if we have conditioned an animal so that by pressing a button it obtains food, but it only falls into the cage when a light is on, the light on is the discriminative stimulus. That is, that stimulus to which the organism has identified that its occurrence implies the probability of obtaining a punishment or reinforcement, this according to the emission of different past behaviors (learning history).

The operant conditioning as we know it was formulated and systematized by Burrhus Frederic Skinner based on the ideas previously raised by other authors.

Ivan Pavlov and John B. Watson had described classical conditioning, also known as simple or Pavlovian conditioning.

For his part, Edward Thorndike introduced the law of effect, the clearest antecedent of operant conditioning. The law of effect states that if a behavior has positive consequences for those who perform it, it will be more likely to be repeated, while if it has negative consequences, this probability will decrease.