A developer needs to run a simple query over multiple comma-separated (.csv) report files stored in Amazon S3 and produce a summary with the least operational overhead. Which service should they use?

Amazon Athena is the best choice because it allows you to directly query data stored in Amazon S3 using standard SQL without provisioning or managing any servers, minimizing operational overhead. It's ideal for ad-hoc analysis of data in various formats, including CSV. Amazon S3 Select allows querying data directly within S3 objects, but it's designed for filtering and retrieving subsets of data from a single object, not for complex queries across multiple files or producing aggregated summaries as efficiently as Athena. Amazon Redshift is a data warehouse service optimized for complex analytical queries on large datasets, but it requires provisioning and managing clusters, which increases operational overhead. Amazon EC2 provides virtual servers, offering complete control but requiring the user to install, configure, and manage all database software and infrastructure, leading to significant operational overhead.