Amazon S3 Glacier is the correct answer because it is a secure, durable, and low-cost storage class specifically for data archiving and long-term backup. It provides retrieval times ranging from a few minutes to several hours, aligning with the company's accessibility requirement. This makes it the most cost-efficient choice for storing data that is infrequently accessed but must be retained. Amazon S3 Standard is designed for data that is accessed more frequently, which incurs higher costs, making it unsuitable for storing rarely accessed backups. Amazon RDS snapshot feature is intended for databases managed by the RDS service, not for archiving on-premises data. AWS Storage Gateway's Tape Gateway simulates a physical tape infrastructure, which might be more complex and less cost-efficient compared to a straightforward archiving service like S3 Glacier for the described scenario.