A company runs an application on Amazon EC2 instances and needs a disaster recovery solution with a recovery time objective (RTO) of less than 4 hours. The solution must use the fewest possible AWS resources during normal operations. Which solution meets these requirements in the MOST operationally efficient way?

The correct answer describes a pilot light disaster recovery strategy, which is cost-effective and meets the RTO. Creating AMIs and copying them to a secondary region, combined with CloudFormation for automated infrastructure deployment, allows for rapid recovery without maintaining active, duplicate resources. CloudFormation provides an Infrastructure as Code (IaC) solution, making it highly operationally efficient for deploying an entire environment. The option using AWS Lambda and custom scripts is less operationally efficient than CloudFormation for deploying an entire infrastructure stack, as CloudFormation is purpose-built for this. Launching active EC2 instances in a secondary region or Availability Zone (hot standby) would incur higher costs during normal operations, violating the "fewest possible AWS resources" requirement.