Which of the following is designed to ensure high availability of web based applications?

Proxies forward client requests to servers and can provide caching or filtering benefits, but they do not distribute load across multiple servers or provide failover for high availability.

Load balancers operate at Layers 4 or 7 of the OSI model to distribute client requests across a pool of backend servers. By detecting server failures via health checks and rerouting traffic to healthy nodes, they eliminate single points of failure and ensure web applications remain available even if individual servers go offline, directly addressing high availability requirements.

URL filtering is a security control that allows or blocks web requests based on categorized URLs, with no role in application availability or traffic distribution.

Routers direct traffic between networks at Layer 3 and are not designed to distribute application-layer requests across multiple servers for availability purposes.