Final answer:
A VR-5 schedule of reinforcement refers to a variable ratio schedule where a subject receives reinforcement on average every 5 trials, which means that the number of responses required for reinforcement is unpredictable but averages out to five.
Step-by-step explanation:
A VR-5 schedule of reinforcement means that reinforcement is provided after an unpredictable number of responses, with an average of five responses needed before reinforcement is given. Specifically, option 2 is correct: A subject receives reinforcement on average every 5 trials. This is known as a variable ratio schedule, which is a type of partial or intermittent reinforcement schedule that is particularly resistant to extinction and promotes a high and steady rate of response because the subject doesn't know exactly which response will be reinforced, but they do know that, on average, every five responses will be rewarded.
In contrast to this, the fixed ratio schedule delivers reinforcement after a predictable number of responses, and a fixed interval schedule provides reinforcement after a set amount of time. Variable interval schedules, on the other hand, deliver reinforcement based on varying and unpredictable amounts of time. Among all these schedules, the variable ratio schedule, like the VR-5, tends to be the most effective in maintaining the behavior over time.