A developer registered a Lambda function as a target for an Application Load Balancer (ALB) using the CLI, but the Lambda is not invoked when requests hit the ALB. Why is the Lambda not being called?

The correct answer is that the Lambda function is missing the required permissions to allow invocation by the ALB. When an ALB invokes a Lambda function, it acts as a principal that needs permission to call the lambda:InvokeFunction API action. This permission is granted via a resource-based policy attached to the Lambda function. Without this policy, the ALB cannot invoke the function, even if it's correctly registered as a target. Lambda functions can be registered as targets for an ALB, so the first option is incorrect. While the AWS Management Console can be used, registration via the CLI is also fully supported, making the second option incorrect. Cross-zone load balancing is a feature for distributing traffic across multiple Availability Zones for EC2 instances, not directly related to Lambda invocation by an ALB, so the fourth option is incorrect.