Which AWS service provides a fully managed database service for applications?

The choice of Amazon RDS as the correct answer is based on its function as a fully managed relational database service that simplifies the setup, operation, and scaling of databases in the cloud. Amazon RDS (Relational Database Service) automates administrative tasks such as hardware provisioning, database setup, patching, and backups, allowing developers to focus on their application development rather than managing database infrastructure.

Amazon RDS supports several database engines, including MySQL, PostgreSQL, MariaDB, Oracle, and Microsoft SQL Server. This versatility makes it suitable for a variety of use cases. Furthermore, RDS offers scalability options where you can easily resize your database instance and use storage that dynamically scales with your needs.

In contrast, AWS Lambda is a serverless computing service that executes code in response to events, but it is not a database service. Amazon EC2 is a compute service that allows you to launch virtual servers in the cloud, where you would have to manage your own databases manually. Amazon S3 is an object storage service designed for storing and retrieving large amounts of data rather than managing relational databases.

Therefore, the correct choice is Amazon RDS, as it uniquely fulfills the requirement of providing a fully managed database service specifically tailored for applications.