the three-term contingency becomes the four-term contingency if we add reinforcers. so reinforcers is correct answer of given question.
In operant conditioning, a reinforcer is a stimulus that is administered or altered after the intended response in order to improve the likelihood of the desired response (for example, a reward or the removal of an electric shock). The least contrived activities or things that nonetheless effectively promote the behavior are the best reinforcers. For instance, if a youngster finds both jumping on the trampoline and drawing pictures to be reinforcing, drawing images might be a more suitable reinforcer for doing their homework.
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