A Security Analyst Wishes to Increase the Security of an FTP

A security analyst wishes to increase the security of an FTP server. Currently, all traffic to the FTP server is unencrypted. Users connecting to the FTP server use a variety of modern FTP client software. The security analyst wants to keep the same port and protocol, while also still allowing unencrypted connections. Which of the following would BEST accomplish these goals?

A) Require the SFTP protocol to connect to the file server.
B) Use implicit TLS on the FTP server.
C) Use explicit FTPS for connections.
D) Use SSH tunneling to encrypt the FTP traffic.