Answer:
Operant conditioning.
Explanation:
As per the given description, the situation best illustrates 'operant conditioning' as the achieved reward or goal('ability to use vending machine') promotes the constant display of such behavior('frequent use of vending machines') by associating it to the reward(accomplishment of goal) received for such action the previous time. Skinner's theory is associated with promoting 'learning via association' as an individual could promote or deny a particular behavioral response by associating it to reward or punishment received previously for such action. Thus, 'operant conditioning' is the answer.