Final answer:
Slot machines operate on a variable ratio reinforcement schedule, making the reward system unpredictable and consequently quite addictive, as gamblers never know when they will win next. As such, the correct answer to the question of which reinforcement schedule slot machines operate on is b. variable ratio.
Step-by-step explanation:
Slot machines reward gamblers with money according to a variable ratio schedule of reinforcement. This means that a reward is delivered after an unpredictable number of responses, which could vary each time a person plays. The reason this type of schedule is particularly potent and addictive is that it creates a high level of persistence in behavior, as it is unknown when the next reward will come, much like a gambler's win on a slot machine.
Skinner highlighted the power of variable-ratio reinforcement in maintaining gambling behavior, as it leads to a high and steady response rate without knowing when the next reinforcement will occur. It is this unpredictability combined with the possibility of reward that keeps people engaged in the activity, continuously hopeful for the next win.