Which type of reinforcement schedule reinforces behavior after an unpredictable number of responses?

The correct choice relates to the concept of reinforcement schedules in operant conditioning. A variable-ratio schedule reinforces a behavior after an unpredictable number of responses, which creates a high level of responding. This unpredictability keeps the individual engaged in the behavior because the reward can come at any moment after a number of responses that cannot be precisely predicted.

For example, this type of schedule is often observed in gambling scenarios where a player may win after a varying number of bets. Because the requirements for obtaining the reinforcement (e.g., a reward or prize) change unpredictably, individuals tend to persist in their behavior, leading to high rates of response.

Other reinforcement schedules operate on different principles: fixed-interval schedules provide reinforcement after a set amount of time, variable-interval schedules deliver reinforcement after varying time periods (but still based on the passage of time), and fixed-ratio schedules reward behavior after a specific number of responses. Thus, while these other schedules can also reinforce behavior, they do not align with the definition of reinforcing after an unpredictable number of responses like the variable-ratio schedule does.