A stimulus change that increases the future frequency of behavior that immediately precedes it.

• A. Reinforcer
• B. Reinforcement
• C. Relevance of Behavior Rule
• D. Reactivity

Answer: (A)

The key idea here is identifying the specific stimulus that increases how often a behavior occurs in the future. A reinforcer is that stimulus: it’s the consequence that follows a behavior and makes it more likely to happen again. This can be added after the behavior (positive reinforcement) or, if removed after the behavior, also increases its likelihood (negative reinforcement). Reinforcement, in contrast, refers to the process itself—the effect of delivering or withholding a stimulus after a response—so the term that names the actual stimulus is reinforcer. The other terms don’t describe the stimulus: reinforcement is the process, the Relevance of Behavior Rule covers when behavior is maintained by consequences, and reactivity is any change in behavior due to being observed.