What type of encryption is best suited for use on the file servers for the proprietary data, and how might you secure the data when it is in motion?

AES is a strong modern symmetric encryption algorithm that is appropriate for encrypting data at rest. TLS is frequently used to secure data when it is in transit. A virtual private network is not necessarily an encrypted connection and would be used for data in motion, while DES is an outdated algorithm and should not be used for data that needs strong security.