Stopping the DB instance is the most cost-effective option among the choices because it preserves the database for later restart without continuously paying for compute. AWS documents that a stopped RDS DB instance retains its metadata and can be restarted when needed, and that stopped instances automatically restart after 7 days. In practice, this means the company can periodically stop and restart the instance as required for short testing windows. The other options introduce unnecessary rebuild work or architecture changes. While the question wording includes both stop-db-cluster and stop-db-instance, the practical intent is clearly to stop the RDS MySQL instance temporarily and restart it when needed.

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