Answer: Variable-interval
Explanation:
Variable-interval schedule is the reinforcement schedule in which reinforcement is implied to response after certain passage of time. This time is usually unpredictable as variable schedule.
According to the question, variable interval schedule is reinforced through Professor Wallace as he is going to give pop quizzes to students at uncertain time of interval which is not predictable or has varying schedule.
Other options are incorrect because fixed ratio schedule, variable ratio schedule and fixed interval schedule are not schedule on which Professor's surprise quiz will reinforce.Thus, the correct option is variable interval schedule.