A company will store 5 MB audio files in Amazon S3, rarely access them, but must be able to retrieve them immediately. Which S3 storage class is the most cost-effective fit?

S3 Standard-Infrequent Access (S3 Standard-IA) is the most cost-effective choice because it's designed for data that is accessed less frequently but requires rapid retrieval when needed. For rarely accessed 5 MB audio files, the lower storage cost of S3 Standard-IA outweighs its per-GB retrieval fee. S3 Standard is more expensive for storage and is for frequently accessed data. S3 Glacier Flexible Retrieval and S3 Glacier Deep Archive offer even lower storage costs but have retrieval times ranging from minutes to hours, which does not meet the requirement for immediate retrieval.