A: Creating an export job on the first of the month and saving the export file to a Cloud Storage bucket with the Archive class is a good approach. It allows for manual export on a schedule and uses Archive storage for cost efficiency. However, it lacks automatic retention enforcement (auto-delete after 3 years).

B: Relying on the automatic first-of-the-month backup and setting the bucket object to auto-delete after three years, then storing the backup in Cloud Storage Archive class is problematic. Automatic backups are managed by Cloud SQL and cannot be exported as files for user management, making this option incorrect in practice.

C: Setting up an on-demand backup on the first of the month and writing the backup to Cloud Storage Archive class is incorrect because Cloud SQL backups are internal and cannot be directly written to Cloud Storage by the user. A separate export to Cloud Storage is required.

D: Attempting to convert an automatic first-of-month backup to an export file and saving it to Cloud Storage Coldline class is incorrect. Automatic backups cannot be converted to exports; exports must be performed explicitly. Additionally, Coldline class is more expensive than Archive for long-term storage.