Final answer:
Mrs. Graham uses a schedule in operant conditioning by having unpredictable pop quizzes to encourage students to study regularly, reinforcing the behavior of consistent studying.
Step-by-step explanation:
Mrs. Graham enforces a desired behavior in her students using a schedule in operant conditioning (answer c), specifically by implementing pop quizzes. This approach is based on the lack of predictability as to when the quizzes will occur, encouraging students to study regularly rather than only just before scheduled tests. Operant conditioning seeks to modify behavior through reinforcement or punishment. In this case, the reinforcement comes from students potentially achieving better grades on pop quizzes due to more consistent studying habits.
Answer: c) Using a schedule in operant conditioning