Which of the following is an example of a primary reinforcer?

A primary reinforcer is something that inherently satisfies a biological need and is naturally reinforcing. Food is a quintessential example of this because it fulfills the fundamental need for sustenance and survival. When an individual engages in a behavior and is subsequently given food as a reward, this increases the likelihood of that behavior being repeated in the future due to the direct biological satisfaction it provides.

In contrast, the other options, while they can serve as reinforcers, do not fulfill a basic biological need in the same immediate way. Money, for instance, is a secondary reinforcer; its value is derived from the ability to use it to obtain primary reinforcers such as food or shelter. Praise is also a form of social reinforcement but is contingent on social contexts and does not directly satisfy physical needs. Tokens are symbolic representations that can be exchanged for rewards, but they do not carry intrinsic value on their own like food does.