The pattern of partial reinforcement in which responses are regularly reinforced after a set period of time has passed is called a schedule.

Fixed interval

Explanation:

Reinforcement schedules are the guidelines which decide how often an organism is reinforced for a specific behavior. A fixed-interval schedule is used in operant conditioning and it is a reinforcement schedule where the initial response is only rewarded after a given amount of time has passed. While in a partial schedule only some of the behavioral instances are improved, and not every occasion. Behaviors are formed and developed more slowly through a partial reinforcement schedule.

i would say b probably

can you explain it better