Which AWS service provides a fully managed message broker?

The correct answer is tied to the service that offers a fully managed message broker, which is specifically designed to facilitate communication between different applications or services through message queuing. Amazon MQ serves as a managed message broker that makes it easy to set up and operate message brokers for various protocols such as MQTT and AMQP. It allows developers to focus on building applications rather than managing the underlying infrastructure, thereby providing the reliability and scalability needed for messaging applications.

In contrast, AWS Lambda is a serverless compute service that allows you to run code in response to events, but it does not function as a message broker. Amazon Simple Notification Service (SNS) is a pub/sub messaging service aimed at sending notifications to multiple subscribers but does not manage the message brokering aspect of message queues or topics. Similarly, AWS Simple Queue Service (SQS) provides queuing capabilities to decouple and scale microservices and distributed systems, yet it differs from a full message broker in that it does not support the breadth of messaging patterns and protocols that Amazon MQ does.

Thus, Amazon MQ stands out as the correct option due to its specific role as a fully managed, feature-rich message broker that supports various messaging protocols and integration capabilities.