The correct answer is A. Configuring the fleet of EC2 instances to use encrypted EBS volumes for data storage directly fulfills the encryption requirement while minimizing performance impact. AWS EBS encryption provides encryption at rest and integrates seamlessly with AWS Key Management Service (KMS) for key management. This process is transparent to the applications utilizing the EBS volumes, ensuring minimal performance degradation. Other options, such as writing data to an encrypted Amazon S3 bucket (Option B), custom application encryption (Option C), or using an encrypted root volume and ephemeral disks (Option D), may introduce additional complexities, latencies, or are less optimal for this specific use case.