Which network device would you use to ensure that traffic continues to flow to a functional web server when another server in a cluster goes down?

In this scenario, the most appropriate choice is a load balancer. A load balancer is specifically designed to distribute incoming network traffic across multiple servers, ensuring that no single server becomes overwhelmed and that the service remains available even if one of the servers in the cluster fails.

When a web server in a cluster goes down, the load balancer can automatically reroute traffic to the remaining functional servers, maintaining continuous service for users. This capability enhances redundancy and improves overall system reliability.

In contrast, a router primarily directs data packets between networks and does not provide load balancing capabilities as a core function. A firewall is focused on network security by controlling incoming and outgoing traffic based on predefined security rules, and while it may be involved in traffic direction, it does not balance the load across multiple servers. A NAT gateway, which translates private IP addresses to a single public IP address, provides address translation rather than balancing traffic among servers. Therefore, the load balancer is clearly the best choice for maintaining uptime and performance in a clustered web server scenario.