Which AWS service or feature should the developer use to meet these requirements with the LEAST amount of operational overhead?

Explanations:

Amazon S3 Select allows for querying data stored in S3 files but is limited to retrieving a subset of data from individual files. It does not provide a way to aggregate data across multiple CSV files efficiently.

Amazon Athena is a serverless query service that allows users to run SQL queries directly on data stored in S3. It can easily query multiple CSV files and generate summary reports with minimal operational overhead, making it the most suitable option for this scenario.

Amazon Redshift is a data warehousing service that requires setup and management of a cluster, which introduces more operational overhead compared to Athena. It is not ideal for ad-hoc querying of CSV files in S3 without significant data integration work.

Amazon EC2 involves setting up and managing virtual servers, which requires significant operational overhead for running queries on CSV files in S3. This is not the best choice for simply querying data stored in S3.