Explanation: A full-duplex VHF radiotelephone transmits and receives simultaneously. The most critical consideration is preventing the powerful outgoing transmit signal from overwhelming or "desensitizing" the sensitive receiver, which needs to hear much weaker incoming signals. Therefore, maximizing the physical separation between the transmitting antenna and the duplex receiver antenna (D) is paramount. Greater physical distance provides more "path loss" for the transmit signal before it reaches the receiver antenna, ensuring the receiver isn't overloaded and can effectively detect distant stations. Option A refers to the radio units, not the antennas, and while relevant for cabling, it's not the primary RF issue. Options B and C relate to multiple transmitting antennas, which isn't the primary challenge for a *single* full-duplex radio setup. The core issue is isolating *its own* transmit from *its own* receive.