Which AWS service is primarily used for querying large datasets stored in S3?

Amazon Athena is the correct choice for querying large datasets stored in Amazon S3. It is an interactive query service that allows users to analyze data directly in S3 using standard SQL, without needing to set up or manage any infrastructure. This serverless service enables quick querying of structured and semi-structured data stored in various formats, including CSV, JSON, and Parquet.

One of the significant advantages of using Amazon Athena is its ability to scale automatically to respond to query workloads, which is particularly beneficial when dealing with large datasets. Additionally, users pay only for the queries they run, making it cost-effective for ad-hoc analysis.

Other AWS services have their own functions but are not primarily focused on querying data in S3 in the same manner. Amazon EMR is primarily used for processing huge volumes of data using big data frameworks such as Apache Hadoop and Spark, requiring more setup. Amazon RDS is designed for relational database management and does not directly query files in S3. Amazon QuickSight is a business intelligence tool used for data visualization and reporting but relies on data sources, such as Athena or RDS, rather than querying data directly from S3 itself.