The correct option is D. Using AWS Lambda functions in conjunction with Amazon EventBridge (CloudWatch Events) to automatically start and stop the DB instance based on a schedule is a cost-effective solution. This approach ensures that the DB instance runs only during the required 12 hours daily, thereby minimizing costs associated with running the database instance for the entire day. Options A, B, and C involve either additional complexity or do not directly address the need to control DB instance costs effectively.