Question

Which protocol and a data exchange system commonly used over tcp/ip networks, including the internet, but which is unencrypted and performs authentication and data transfer in plaintext?

Answer 1

There are a quite a few TCP protocols that don't support encryption (probably more than those that do).

Telnet (no encrypted variant, closest thing to an encrypted Telnet is SSH)

FTP (has an encrypted variant, FTPS)

HTTP (has an encrypted variant, HTTPS)

IMAP, (has an encrypted variant, IMAPS)

SMTP, (has an encrypted variant, SMTPS)