Which AWS service is used to run code in response to events without provisioning servers?

AWS Lambda is the correct choice because it is designed specifically for running code in response to events without the need for managing servers. It operates on a serverless architecture, meaning that developers can focus solely on writing the code while AWS handles the infrastructure. This allows users to execute functions automatically when certain triggers occur, such as changes to data in an S3 bucket or updates in DynamoDB, among other event sources.

With AWS Lambda, users benefit from robust scalability and cost-efficiency since they only pay for the compute time consumed. This aligns perfectly with the modern event-driven architectures that many applications are moving towards, avoiding the overhead associated with server management.

In contrast, Amazon EC2 requires users to provision and manage virtual machines, AWS Fargate is a serverless compute engine for containers that still involves defining a task execution environment, and Amazon ECS is a container orchestration service that still necessitates some level of infrastructure management. Thus, AWS Lambda stands out as the best option for executing code in response to events without provisioning or managing servers.