It is difficult to predict when a slot machine will payoff. Sometimes it takes 200 pulls, sometimes 500, sometimes 1,000 or more. Since you know it must payoff sometime, you keep plugging in the coins. Slot machines like this are set up on:

A. Fixed-ratio schedule of reinforcement
B. Variable-ratio schedule of reinforcement
C. Fixed-interval schedule of reinforcement
D. Variable-interval schedule of reinforcement

My first answer was d but this was wrong. Is the correct answer b?

Yes, the correct answer is B. Variable-ratio schedule of reinforcement.

A variable-ratio schedule of reinforcement is a type of operant conditioning schedule in which reinforcement (in this case, the slot machine payoff) is provided after an unpredictable number of responses (pulling the lever). This schedule is known for producing high response rates and persistence in behavior. It is similar to how casinos set up their slot machines to keep players engaged and motivated to continue playing, as they are uncertain when the next payoff will occur.