Final answer:
The humanities instructor is using a d. variable-interval schedule of reinforcement to encourage students to consistently keep up with their reading and attend all classes.
Step-by-step explanation:
The humanities instructor from the question is using a variable-interval schedule of reinforcement to encourage studying and class attendance. In a variable-interval schedule, reinforcements are given after unpredictable time intervals which keeps the students on their toes, as they do not know when the next quiz (reinforcement) will be administered. This strategy promotes a moderate but steady response rate from the students, as they need to be prepared at all times for a potential surprise quiz.