Communication becomes circular when

Concept — what makes a communication model circular. Communication models are classified by the direction in which the message flows between the two parties. In a LINEAR model the flow is one-way and the roles are fixed: a source encodes a message, a channel carries it, and a receiver decodes it, and those two roles never swap. A model is CIRCULAR when the path of the message closes back on itself, i.e. when the same participant performs BOTH functions in turn — the party who has just decoded an incoming message now encodes a message of its own and transmits it, and the original sender in turn decodes that. Osgood and Schramm described exactly this arrangement: each participant is simultaneously encoder, interpreter and decoder, so the process has no fixed beginning and no fixed end. The returning message produced by that role reversal is what we ordinarily call feedback.

Application to this item. The condition asked for is the structural one that closes the loop, and that condition is met when the decoder becomes an encoder. Reading it step by step:

1. The source encodes a message and sends it; the receiver decodes it. At this point the flow is still a straight line from one party to the other.

2. The receiver stops being a passive endpoint and encodes a message of its own — it takes on the encoding function that until now belonged only to the source.

3. That newly encoded message travels back and is decoded by the original source, which is now acting as receiver.

4. Both parties now perform both functions, so the message path has closed into a loop. The process is circular.

Cross-check against the other conditions offered. Each of the remaining conditions describes a real property of a communication act, but none of them changes the DIRECTION of flow, which is the only thing circularity is about:

• “the feedback is absent” — if no return message travels from receiver to source, the return leg does not exist at all. That is the defining shape of a one-way, linear process such as the Shannon–Weaver transmission model. Removing feedback moves a model away from circularity, not towards it; and feedback is itself a CONSEQUENCE of the receiver encoding, not a separate mechanism.

• “the source is credible” — source credibility (classical rhetoric calls it ethos) is a judgement the audience makes about the sender's expertise and trustworthiness. It affects how persuasive a message is, but a highly credible source can broadcast in one direction indefinitely; credibility is a property of the sender, not of the direction of flow.

• “the channel is clear” — a clear channel means the medium is free of physical or semantic noise, so the signal received closely matches the signal sent. That governs the FIDELITY of transmission, not its shape: a message can cross a perfectly clear channel in one direction only and the process remains linear.

Result. Communication becomes circular when the decoder becomes an encoder — the role reversal that turns a one-way line into a closed loop.