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 you to retrieve a subset of data from an object stored in S3, but it is primarily designed for querying specific data within a single file rather than aggregating data across multiple files. It does not support complex queries or generate summary reports efficiently.

Amazon Athena is a serverless interactive query service that allows you to analyze data directly in Amazon S3 using standard SQL. It can easily handle multiple CSV files stored in S3, enabling the developer to write a simple query to generate summary reports with minimal operational overhead.

Amazon Redshift is a fully managed data warehouse service that requires more setup and operational management. It is not the best choice for simply querying CSV files in S3 without additional ETL processes. Redshift would also incur additional costs and complexity compared to Athena.

Amazon EC2 provides virtual servers to run applications, but using EC2 for querying CSV files in S3 would require additional setup, management, and maintenance. This option involves significantly higher operational overhead compared to the serverless and managed options like Amazon Athena.