Questions: 4. Which strategy is the most efficient for Amazon EC2 disaster recovery (DR)? Synchronize instances with standby instances on nearly a continuous basis. Rebuild instances by using Amazon Machine Images (AMIs) from AWS Marketplace. Store essential data separately from the instance, and develop rapid rebuild processes for compute instances. Back up instances on a regular schedule.

The answer is the third one: Store essential data separately from the instance, and develop rapid rebuild processes for compute instances.

Explanation for each option:

1. Synchronize instances with standby instances on nearly a continuous basis: This strategy, known as active-active or hot standby, can be very efficient but is also typically more expensive due to the need for continuous synchronization and the cost of running standby instances.

2. Rebuild instances by using Amazon Machine Images (AMIs) from AWS Marketplace: While this can be a part of a disaster recovery strategy, relying solely on AMIs from the AWS Marketplace may not be the most efficient approach. It can be time-consuming to rebuild instances from scratch, and there may be additional configuration required.

3. Store essential data separately from the instance, and develop rapid rebuild processes for compute instances: This is often the most efficient strategy. By decoupling data from the compute instances and storing it in a durable storage service like Amazon S3 or Amazon EBS, you can ensure data persistence. Rapid rebuild processes, such as using infrastructure as code (IaC) tools like AWS CloudFormation or Terraform, can quickly recreate the compute environment, minimizing downtime.

4. Back up instances on a regular schedule: Regular backups are important for disaster recovery, but they alone may not be the most efficient strategy. Restoring from backups can be time-consuming, and there may be data loss depending on the backup frequency.

Summary: The most efficient strategy for Amazon EC2 disaster recovery is to store essential data separately from the instance and develop rapid rebuild processes for compute instances. This approach ensures data durability and allows for quick recovery of the compute environment.