To qualify as a reinforcer,a reward must:

A) involve money
B) involve money and/or praise
C) increase the frequency of the behavior it follows
D) cause extinction

A reinforcer is any event that follows a behavior and increases the likelihood of that behavior being repeated in the future. In other words, it strengthens the behavior. Money and praise are commonly used as reinforcers, but they are not necessary. The key criterion for a reward to be a reinforcer is that it increases the frequency of the behavior it follows. Therefore, choice C is the best option as it correctly identifies the key criterion of a reinforcer. Choices A and B are incorrect as they suggest that money or both money and praise are required for something to be a reinforcer. Choice D is incorrect as it confuses the concept of a reinforcer with that of extinction, which involves the reduction or elimination of a behavior by removing the reinforcing consequences that were maintaining it.