Answer: Fixed interval
Explanation: Reinforcement schedule explains when instances of a behavior will be reinforced, that is the rate or timing of a reinforcement.
In the scenario above, the instructor is employing a fixed interval schedule with his decision to conduct a test every three weeks till the end of the semester. Fixed interval schedule are usually set, such that the date for upcoming reinforcements are already known. And it is not dependent on the outcome of the reinforcement.
