Final answer:
Alex received money, a secondary or conditioned reinforcer, for his work, which is key in operant conditioning for behavior modification.
Step-by-step explanation:
The payment Alex received for his work is an example of a secondary reinforcer. A secondary reinforcer, such as money, does not have inherent value but gains its reinforcing properties through its association with primary reinforcers, like food and water. In operant conditioning, which was developed by psychologist B.F. Skinner, behaviors are influenced by the consequences that follow them. This kind of reinforcement is critical because it strengthens the likelihood of the behavior recurring in the future.
The correct answer to the question, "Alex was paid $100 for eight hours of work. The money was a(n)", is C. conditioned reinforcer. Conditioned or secondary reinforcers like money are powerful tools in behavior modification and are used in various settings, including workplaces and educational institutions, to encourage desirable behaviors.