What condition is necessary for successful full-duplex communication?

Successful full-duplex communication allows both parties to send and receive messages at the same time, which means they can speak and listen simultaneously without any interruption. This capability is essential in a variety of communication systems, such as telephones or radio systems, where real-time interaction enhances the flow of conversation and reduces delays. In full-duplex mode, the channels for sending and receiving signals are separate, enabling a more dynamic dialogue.

The other options do not align with the principles of full-duplex communication. If users were required to take turns speaking, it would resemble half-duplex communication, where only one party can transmit at any given time. Limiting communication to one user at a time or requiring physical proximity eliminates the very advantage of a full-duplex setup, which is designed to facilitate ongoing and interactive exchanges. Thus, simultaneous speaking by both users is the defining characteristic that makes full-duplex communication effective and efficient.