Which of the following centralized access control mechanisms is not appropriate for mobile workers access the corporate network over analog lines?

I have doubts about this question.
On other foruns I saw different answers.

“The de facto communication standard for wireless LANs is spread spectrum, a wideband radio frequency
technique originally developed by the military for use in secure, mission-critical communications systems.” –
Ronald Krutz The CISSP PREP Guide (gold edition) pg 153

However encryption is the best defense when a transmission is captured because it is hard to decrypt.
I have no idea what this question is looking for but I am going with A (frequency hopping).
But D (encryption was the original answer) and some people are sure that it´s C (Optical Fiber)

So:
For wireless interception: A – Frequency hop
For Physical interception: B – Optical Fiber
For Message content interception: D – Encryption