An insurance company archives transaction records compressed with gzip and needs to run occasional queries for audits in the most cost-effective way. Which approach should they use?

Storing the gzipped transaction records in Amazon S3 and using S3 Select is the most cost-effective approach for occasional queries. S3 Select allows you to retrieve only a subset of data from an object by using simple SQL expressions, significantly reducing the amount of data transferred and thus lowering costs. Since the data is already gzipped, S3 Select can directly query compressed files. Storing data in Amazon Glacier Flexible Retrieval (formerly S3 Glacier) or Glacier Instant Retrieval would incur retrieval costs and delays that are not ideal for even "occasional" queries, especially when compared to S3 Standard. While Athena can query data in S3, it is typically more expensive for simple, occasional queries on individual files compared to S3 Select, as Athena processes the entire dataset or partitions, and charges based on data scanned.