Which of the following examples illustrates an unconditioned reinforcer?

An unconditioned reinforcer, also known as a primary reinforcer, is a stimulus that is inherently reinforcing because it satisfies a basic biological need. Food exemplifies this type of reinforcer, as it is essential for survival and does not require any prior learning or conditioning to be effective. The need for food is an innate drive that all humans and many animals share, making it a powerful tool for reinforcing behaviors without any additional context.

In contrast, money, compliments, and tokens are considered conditioned or secondary reinforcers. These reinforcers acquire their value through association with other reinforcers or through social or cultural context. For instance, money is valuable because society has agreed to use it as a medium of exchange. Compliments can reinforce behavior by providing social approval, and tokens gain value through their ability to be exchanged for primary reinforcers or privileges. Therefore, food stands out as the correct answer as it directly fulfills a biological need and acts as an unconditioned reinforcer.