Which solution will accomplish this goal with the LEAST operational overhead?

Explanations:

S3 Storage Lens provides a comprehensive overview of storage usage and access patterns across S3 buckets. It requires minimal operational overhead since it offers advanced activity metrics without the need for additional configuration or analysis tools. This makes it the most efficient choice for identifying rarely accessed or inactive buckets.

The S3 dashboard in the AWS Management Console provides basic information about storage usage and activity but lacks the advanced analytics and metrics necessary for effectively identifying rarely accessed buckets. It also requires manual review and may not provide the depth of insights available in S3 Storage Lens.

While enabling the BucketSizeBytes metric in CloudWatch allows for monitoring the size of buckets, it does not provide direct insights into access patterns. Using Amazon Athena to analyze this data adds complexity and operational overhead, making it less efficient for the goal of identifying infrequently accessed buckets.

Enabling AWS CloudTrail for S3 object monitoring allows tracking of API calls but requires parsing CloudTrail logs to analyze access patterns. This approach involves more operational overhead and complexity compared to S3 Storage Lens, which is designed specifically for storage analysis.