What transmission mode allows two-way communication but only one direction at a time (like a walkie-talkie)?

• A. Half-duplex transmission
• B. Full-duplex transmission
• C. Simplex transmission
• D. Serial transmission

Answer: (A)

Two-way communication where only one direction is active at a time is half-duplex. In this mode the channel is shared and devices can send or receive, but not simultaneously. A walkie-talkie exemplifies this: you press the talk button to transmit, others listen, and when you release, the channel is free for others to transmit.

If both directions could happen at the same time, that would be full-duplex (like a telephone call). If the communication goes in only one direction, that's simplex (one-way broadcast). Serial transmission describes sending bits in sequence and isn’t about whether the path is used in one or both directions.