The correct answer is A. AWS Transfer Family with SFTP for Amazon S3 provides a managed service to transfer files in and out of S3 using the SFTP protocol. This solution is ideal for customers who want to leverage their on-premises Active Directory for authentication while minimizing operational overhead. It also does not require changes to the customer's application, which already uses SFTP for file transfers. Options B, C, and D involve more complex setups or do not directly address the specific requirements as efficiently and straightforwardly as Option A.