Which type of Amazon Elastic Load Balancer operates at layer 7 of the OSI model?

The Application Load Balancer operates at layer 7 of the OSI model, which is the application layer. This type of load balancer is designed to make routing decisions based on the content of the request, allowing for advanced features such as URL-based routing, host-based routing, and support for WebSocket and HTTP/2 protocols.

By functioning at this layer, the Application Load Balancer can inspect the details of the request, including HTTP headers and paths, to intelligently distribute incoming application traffic to different servers. This capability is particularly useful for microservices architectures, where requests might need to be routed to various services based on URL patterns or other request attributes.

In contrast, the other types of load balancers serve different purposes: the Network Load Balancer primarily operates at layer 4 and is best suited for handling millions of requests per second, while the Classic Load Balancer supports both layers 4 and 7 but lacks some of the advanced routing capabilities of the Application Load Balancer. The Gateway Load Balancer is designed specifically for integrating virtual appliances and operates at layer 3. Thus, the unique features and operational level of the Application Load Balancer distinguish it as the correct choice for this question.