Which solution will meet these requirements?

Explanations:

Migrating the file server to an EC2 instance in a public subnet would expose the files to the internet, increasing security risks. While IP restrictions in a security group could limit access, this approach does not provide secure download methods or integration with existing authentication mechanisms.

Migrating to Amazon FSx for Windows File Server allows for integration with on-premises Active Directory, enabling secure access controls for authorized users. AWS Client VPN can provide secure connectivity for remote employees, ensuring confidential files are downloaded securely.

While migrating files to Amazon S3 and using a private VPC endpoint provides a secure method for accessing files, it does not inherently provide user authentication and access controls. Signed URLs can provide temporary access, but without integration with an identity provider, it lacks robust security measures for sensitive data.

Migrating to Amazon S3 with a public VPC endpoint and allowing sign-on with AWS Single Sign-On may expose files to unauthorized access due to the public nature of the endpoint. Public endpoints generally do not meet the requirements for secure access to confidential files, as they are accessible over the internet without sufficient access controls.