A company needs to optimize Amazon S3 costs for an application that generates many non-recreatable files of about 5 MB each, stored in S3 Standard. Files must be retained for 4 years and be immediately accessible. They are frequently accessed during the first 30 days and rarely accessed afterwards. Which solution is MOST cost-effective?

The most cost-effective solution is to transition files to S3 Glacier Instant Retrieval after 30 days. S3 Glacier Instant Retrieval offers immediate access, which is a requirement, and is significantly cheaper than S3 Standard-IA or S3 One Zone-IA for data that is rarely accessed but needs quick retrieval. The 5 MB file size is also well-suited for Glacier Instant Retrieval, which has a minimum billable object size of 128 KB. S3 One Zone-IA is cheaper than S3 Standard-IA but lacks multi-AZ redundancy, which might not be suitable for non-recreatable files. S3 Standard-IA is more expensive than Glacier Instant Retrieval for rarely accessed data. Moving to S3 Glacier Flexible Retrieval after 4 years is incorrect because the requirement is to delete the files after 4 years, and Glacier Flexible Retrieval does not offer immediate access.