Which of the following is considered the least secure means of communication?

The least secure means of communication in this context is radio communication. This method is inherently vulnerable to interception because radio waves can be easily picked up by anyone with the appropriate receiving equipment. There are limited measures available to ensure the confidentiality of messages transmitted over radio frequencies, making it less secure than other forms of communication such as written orders, which can be sealed and securely transported, or face-to-face conversations, which are less likely to be overheard by unintended audiences. The effectiveness of radio communication often depends on the environment and technology in use, but its reliance on broadcast means means that sensitive information is at greater risk of being compromised.