You set up an autoscaling instance group to serve web traffic for an upcoming launch. After configuring the instance group as a backend service to an HTTP(S) load balancer, you notice that virtual machine (VM) instances are being terminated and re-launched every minute. The instances do not have a public IP address. You have verified the appropriate web response is coming from each instance using the curl command. You want to ensure the backend is configured correctly. What should you do?

The correct answer is to ensure a firewall rule exists for load balancer health checks. Google Cloud HTTP(S) Load Balancers perform health checks on backend instances to determine their availability. If these health checks fail, the load balancer assumes the instance is unhealthy and removes it from service, often leading to termination and re-launch by an autoscaling group. Since the instances are serving web responses correctly via curl, the issue is likely the health check traffic not reaching the instances. Health checks originate from specific IP ranges (130.211.0.0/22 and 35.191.0.0/16) and require a firewall rule allowing ingress on the health check port (usually HTTP/80 or HTTPS/443). The other options are incorrect because: Allowing HTTP/HTTPS traffic to the load balancer is necessary for user traffic, but doesn't address the health check issue between the load balancer and the backend instances. Assigning public IPs to instances is generally not recommended for backend services and doesn't solve the health check connectivity problem. Load balancers connect to instances via internal IPs. Creating a tag with the load balancer's name and configuring a firewall rule based on that is not the standard or correct way to allow load balancer health checks. Health checks use specific source IP ranges, not load balancer names as sources.