Which of the following protocols operates without any encryption, making data vulnerable to interception?

Telnet is a protocol that operates without encryption, which means that any data transmitted over a Telnet connection is sent in plaintext. This lack of encryption makes it susceptible to interception by attackers who can easily capture the data being transferred, including sensitive information such as passwords and user credentials.

In contrast, HTTPS is designed to secure data in transit by using encryption, which protects against eavesdropping. ICMP is primarily used for network diagnostics and does not carry data that needs to be encrypted in the same context. While SMTP sends emails without built-in encryption, it is often secured using protocols like STARTTLS; thus, it doesn’t share the inherent vulnerability of plaintext transmission found in Telnet. This distinction makes Telnet particularly vulnerable among the options listed, confirming its classification as a protocol operating without encryption.