The correct answer is Amazon S3 Standard-Infrequent Access (S3 Standard-IA). This storage class is designed for data that is accessed less frequently but requires rapid access when needed. It offers the same durability and high throughput of S3 Standard, but with a lower storage cost, which makes it ideal for the described backup data scenario. It is less expensive than S3 Standard for the storage cost, but it incurs retrieval fees.

Amazon S3 Glacier and S3 Glacier Deep Archive are incorrect answers because they offer very low storage costs and are suitable for long-term archival, but the retrieval time can range from a few minutes to several hours, which does not meet the requirement for quick access. S3 One Zone-IA is also incorrect because, despite lower costs than S3 Standard-IA, it stores data in only one Availability Zone and is thus not recommended for vital data backups for which redundancy is important.