You added an Auto Scaling group and an Application Load Balancer to handle increased traffic for an application running on EC2, but the instances are failing the ALB health checks. What is the best first step to diagnose the problem?

The most effective first step is to confirm the application is listening on the protocol and port configured in the Application Load Balancer (ALB) listener. If the application isn't listening on the expected port or protocol, the health checks will fail, regardless of other settings. Incorrect options: Spanning every AWS Region is not necessary for an Auto Scaling group to pass health checks within a single region and is an unlikely cause of immediate health check failures. ALB listener priority is relevant for routing rules when multiple rules apply, but it doesn't directly affect whether an instance passes a health check. A health check failure implies the target isn't responding, not that the rule isn't being hit. The Auto Scaling group's maximum desired instance count affects how many instances can launch, but not whether individual launched instances pass health checks once they are running.