Which of these is an example of an unconditioned reinforcer?

An unconditioned reinforcer is something that naturally reinforces a behavior without any prior learning or conditioning. Food is a prime example of an unconditioned reinforcer because it satisfies a basic biological need. It does not require any learning process for an individual to find it rewarding; the enjoyment and necessity of food are inherent to survival.

In contrast, the other options are conditioned reinforcers. Tokens, money, and certificates of achievement are all learned stimuli that acquire their reinforcing properties through association with other rewards. For instance, money can buy food and other goods, and tokens can often be exchanged for items or privileges, but these do not inherently satisfy a biological need on their own. Therefore, food stands out as the correct answer as it is fundamentally reinforcing without the need for prior conditioning.