Answer: Create Amazon Machine Images (AMIs) to back up the EC2 instances. Copy the AMIs to a secondary AWS Region. Automate infrastructure deployment in the secondary Region by using AWS CloudFormation.

## Explanation **Correct Answer: B** **Why Option B is correct:** 1. **RTO < 4 hours**: AWS CloudFormation provides automated infrastructure deployment that can quickly spin up resources in the secondary region when needed. With proper automation, recovery can be achieved within the 4-hour RTO requirement. 2. **Fewest resources during normal operations**: This solution uses AMI backups and CloudFormation templates, which are stored in S3 and don't incur running costs. No active EC2 instances are running in the secondary region during normal operations, minimizing resource usage. 3. **Operational efficiency**: CloudFormation provides infrastructure-as-code capabilities, making deployment repeatable, consistent, and less error-prone than custom scripts. **Why Option A is incorrect:** - While it meets the RTO requirement, using Lambda and custom scripts is less operationally efficient than CloudFormation. Custom scripts require more maintenance, are more error-prone, and lack the declarative infrastructure management that CloudFormation provides. **Why Option C is incorrect:** - This solution keeps EC2 instances active in a secondary region at all times, which violates the requirement to use "the fewest possible AWS resources during normal operations." This approach (pilot light or warm standby) consumes more resources than necessary. **Why Option D is incorrect:** - Using a secondary Availability Zone within the same region doesn't provide true disaster recovery protection against regional failures. Additionally, keeping instances active violates the resource minimization requirement. **Key Concepts:** - **Disaster Recovery Strategies**: This scenario describes a "backup and restore" or "pilot light" approach where infrastructure is not running but can be quickly deployed. - **RTO (Recovery Time Objective)**: The maximum acceptable time for restoring service after a disaster. - **AWS CloudFormation**: Provides infrastructure as code capabilities for automated, repeatable deployments. - **AMI Copying**: AMIs can be copied between regions to maintain backup images in the DR region. This solution balances cost efficiency (minimal resources during normal operations) with recovery speed (RTO < 4 hours) while maintaining operational efficiency through infrastructure-as-code practices.

Author: LeetQuiz Editorial Team