A CloudFormation template successfully launched an EC2 instance in us-east-1 but failed when used in us-west-2. Which of the following is a likely cause of the failure?

The most likely cause of the failure is that the AMI ID referenced in the template does not exist in the us-west-2 Region. AMI IDs are specific to the AWS Region in which they are created. An AMI that exists in us-east-1 will not automatically exist in us-west-2; a different AMI ID, or a copied AMI, would be required for the us-west-2 Region. Resource tags are global and not Region-specific, so they would not cause a Region-specific failure. While a cfn-init script failure could prevent an instance from being fully configured, the question states the launch failed, implying the instance itself didn't provision, which is a prior step to cfn-init execution. IAM users are global resources and not Region-specific.