Answer:
B. Variable-ratio
Step-by-step explanation:
In operant conditioning, there are four types of reinforcement schedules including fixed-ratio, fixed-interval, variable-ratio, and variable-interval.
Variable-ratio schedule: In psychology, the variable-ratio schedule is a reinforcement schedule in whereby a particular response is being reinforced after an uncertain number of responses and creates a high and steady rate of responding. Variable-ratio reinforcement schedule is most resistant to extinction.
Example: Lottery games and gambling.
In the question above, the machines use the variable-ratio reinforcement schedule.