A company is migrating millions of 10 KB files from on-premises to Amazon S3 using AWS DataSync. For the first year, each file will be accessed once or twice and must be immediately available. After one year, files must be archived for at least 7 years. Which solution is the most cost-effective?

The correct solution is the most cost-effective because grouping small files into larger objects significantly reduces DataSync's operational overhead and S3's per-object costs (e.g., PUT requests, monitoring). S3 Glacier Instant Retrieval is suitable for the first year, offering immediate access at a lower cost than S3 Standard-IA, especially for data accessed infrequently. Transitioning to S3 Glacier Deep Archive after one year provides the lowest storage cost for long-term archiving with a 7-year retention. The second option is less cost-effective because S3 Standard-IA is more expensive than S3 Glacier Instant Retrieval for infrequently accessed data, and transitioning to Glacier Instant Retrieval later is redundant. The third option is incorrect because DataSync does not support directly configuring S3 Glacier Instant Retrieval as a destination storage class for individual small files, and migrating millions of small files individually to Glacier Instant Retrieval would incur high per-object costs. The fourth option is less cost-effective due to the high per-object costs of storing millions of small files in S3 Standard-IA and then transitioning them, as well as the higher cost of S3 Standard-IA compared to Glacier Instant Retrieval for the initial period.