Which of the following is NOT listed as an aid to security?

• A. Call signs
• B. Authentication
• C. Code words
• D. Frequency hopping

Answer: (D)

Aids to security are tools that directly support proving who is communicating and protecting the message itself. Call signs help identify the operator or station, authentication verifies the other party’s identity, and code words provide prearranged, sensitive information or permissions to prevent misuse or misinterpretation.

Frequency hopping is a spread-spectrum technique used to make a signal harder to jam or intercept by rapidly changing frequencies. While this approach can enhance privacy and resilience, in many training materials it’s described as a transmission-method for reliability rather than a direct security credential or protective measure. That’s why it isn’t listed as an aid to security in the context of this question, making it the correct choice for NOT being listed.