Public access has been blocked on all corporate S3 buckets. The administrator wants an automated notification if any bucket becomes publicly readable later. What is the most operationally efficient way to accomplish this?

Enabling the managed AWS Config rule s3-bucket-public-read-prohibited is the most operationally efficient solution. AWS Config continuously monitors your AWS resources for compliance with desired configurations. This specific rule automatically detects if any S3 bucket becomes publicly readable and can trigger notifications via an Amazon SNS topic when non-compliant. This approach is fully managed, requiring minimal setup and ongoing maintenance. Running a Lambda on a schedule or implementing a cron job would require custom code development, deployment, and maintenance, making them less efficient. S3 Event Notifications are primarily for object-level events within a bucket, not for changes to the bucket's public access configuration itself.