What best describes synchronous and asynchronous I/O modems?

• A. Synchronous modems require no clock synchronization.
• B. Synchronous modems transmit at a fixed rate without clock signals.
• C. Asynchronous modems require external clock signals.
• D. Synchronous modems must transmit a clock pulse that is used to synchronize the receiver.

Answer: (D)

Timing reference is essential in synchronous I/O. In synchronous mode, data is sent with a shared timing signal that the receiver uses to sample bits at exact intervals. The transmitter must provide a clock pulse (or a clock signal) so the receiver can stay in step and interpret the incoming stream correctly. That’s why describing synchronous modems as requiring a clock to synchronize the receiver is the accurate depiction.

The other statements miss how timing is handled in synchronous systems. Synchronous operation relies on a clock reference, not on transmitting data at a fixed rate without any clock signals. And asynchronous modems do not depend on an external clock line; they frame data with start/stop bits and rely on the receiver’s local clock to time sampling.