To enforce subnet-level isolation by routing traffic from instance-A through a security appliance instance-B located in another subnet, what should you configure?

To enforce subnet-level isolation and route traffic through a security appliance, you need a custom route that takes precedence over the default system-generated subnet route. Creating a more specific route (e.g., for a smaller CIDR range or a single IP address) ensures it's preferred. Applying a tag to instance-A and specifying this tag in the custom route ensures that only traffic originating from instance-A is directed to instance-B (the security appliance) as the next hop. This method allows granular control without disrupting other traffic in the subnet. Deleting the system-generated subnet route is not feasible or recommended, as it would break connectivity within the subnet. Moving instance-B to another VPC with multi-NIC is overly complex for this scenario and doesn't directly address the need to route instance-A's traffic through instance-B within the same VPC. A route without a tag would apply to all instances in the destination range, not just instance-A, failing to achieve the desired specific isolation.