Final answer:
The reinforcer in the example is (d) Earning a grape, as it is the reward that increases the frequency of the monkeys pressing the correct keys.
Step-by-step explanation:
The reinforcer in this example of a learning researcher training capuchin monkeys is (d) Earning a grape.
In the context of operant conditioning, a reinforcer is any stimulus that increases the likelihood of a behavior being repeated. In this case, it's the grape that serves as a primary reinforcer because it is inherently rewarding for the monkeys. This form of learning utilizes positive reinforcement, a process where the frequency of a desired behavior is increased by following it with a reward. As shown in Skinner's experiment with rats, the animals associate pressing a lever—or in our case, the correct keys—with receiving food, thus strengthening the behavior.
Using continuous reinforcement, the researcher successfully teaches the monkeys to press the right keys to earn a grape. The immediacy of receiving the grape after the desired action helps the monkeys make the association between the specific behavior (pressing the correct keys) and the positive outcome (getting a grape).
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