Option B is the correct answer. It involves creating monthly on-demand DynamoDB backups, transitioning them to Amazon S3 Glacier Flexible Retrieval after 6 months, and using an S3 Lifecycle policy to delete backups after 7 years. This approach directly addresses the compliance requirements for taking monthly backups, maintaining 6 months of availability, and retaining them for 7 years, leveraging S3's lifecycle management features efficiently. Other options either do not fully automate the process or unnecessarily complicate the solution.