Answer: variable-ratio
Explanation:
Based on the information given, since getting a $500 airline ticket only happens after one acquire 25,000 miles (spending $25,000 to do so), this is Variable ratio schedule.
Variable-ratio schedule is regarded as a form of reinforcement which occurs when there is a reinforcement of a response when there are different responses that are unpredictable and this lead to a high number of responses. An example is lottery and gambling.