Final answer:
The true statement about a variable-ratio schedule in operant conditioning is that reinforcement occurs after a variable number of responses, which makes this schedule unpredictable and often leads to a high and steady rate of response.
Step-by-step explanation:
The answer to the question regarding which statement is true of a variable-ratio schedule in operant conditioning is B: Reinforcement occurs after a variable number of responses. This type of reinforcement schedule is characterized by providing rewards after an unpredictable number of responses. For example, a gambler at a slot machine receives reinforcement on a variable ratio schedule because the number of attempts before winning varies each time. This schedule typically yields a high rate of response with little pause after reinforcement because the exact number of responses required for reinforcement is unpredictable. This is in contrast to a fixed ratio reinforcement schedule, where a set number of responses must occur before a behavior is rewarded, and a fixed interval reinforcement schedule, where behavior is rewarded after a set amount of time.