Your professor has informed you at the beginning of the term that you will have eight tests--but they will all be unannounced. This is most similar to a reinforcement schedule.

variable interval

Explanation: variable-interval schedule is a type of operant conditioning reinforcement where a response is rewarded after an unpredictable amount of time has passed, furthermore, this is a type of operant conditioning reinforcement in which reinforcement is given to a response after specific period of time has passed but this amount of time is on a changing/variable schedule.

the 4th option is the answer