A company currently uses an on-premises SFTP file transfer solution and is migrating to the AWS Cloud to scale and reduce costs by using Amazon S3. Employees must authenticate with the existing on-premises Microsoft Active Directory (AD) and the company wants to preserve current authentication and file access mechanisms. Which solution meets these requirements with the LEAST operational overhead?

The correct answer is to create an AWS Transfer Family server with SFTP endpoints, select the AWS Directory Service option as the identity provider, and use AD Connector to connect to the on-premises Active Directory. AWS Transfer Family is a fully managed service that supports SFTP, FTPS, and FTP, allowing seamless migration of file transfer workflows to AWS. By integrating with AWS Directory Service and AD Connector, it directly leverages the existing on-premises Active Directory for authentication, preserving current mechanisms with minimal operational overhead. Configuring an S3 File Gateway and creating SMB file shares is incorrect because the requirement specifies SFTP, not SMB, and this solution doesn't directly address the SFTP protocol for the clients. Configuring an Auto Scaling group of EC2 instances to run an SFTP solution is incorrect because it involves managing EC2 instances and the SFTP software, which increases operational overhead compared to a fully managed service like AWS Transfer Family. The last option is partially correct but incomplete; it omits the crucial step of using AD Connector to bridge the AWS Directory Service with the on-premises Active Directory, which is essential for preserving existing authentication.