A security team requires a way to prove that AWS CloudTrail log files have not been modified after delivery. The organization already uses IAM to restrict who can access particular trails. What is the MOST operationally efficient method to guarantee and validate the integrity of each delivered CloudTrail file?

The most operationally efficient method is to enable CloudTrail's built-in file integrity validation. CloudTrail automatically creates digest files that contain a hash of the log files delivered to your S3 bucket. These digest files are signed by AWS, providing cryptographic proof that the log files have not been tampered with. The security team can then use these digest files to validate the integrity of each delivered CloudTrail file. This is a native, fully managed feature requiring minimal configuration and maintenance. The other options involve custom solutions (Lambda functions, DynamoDB, S3 object tags) which are less efficient because they require developing, deploying, and maintaining custom code and infrastructure. While technically possible, they introduce unnecessary operational overhead compared to CloudTrail's native functionality.