Which statement is true about the CSMA/CD access method used in Ethernet?

• A. A jamming signal causes only devices that caused the collision to execute a backoff algorithm.
• B. Devices involved in a collision get priority to transmit after the backoff period.
• C. When a device hears a carrier signal and transmits, a collision cannot occur.
• D. All network devices must listen before transmitting.

Answer: (D)

CSMA/CD relies on listening before transmitting on a shared Ethernet medium. Each device must sense the channel before it starts sending; if the channel is idle, it may transmit, but because several devices can become ready at similar times, a collision can occur when two transmissions overlap. A jam signal ensures all participants detect the collision, after which the transmitting devices stop and wait a random period (backoff) before trying again. Because the process depends on every node listening and only transmitting when the channel is free, the statement that all network devices must listen before transmitting is the true one. The other ideas don’t fit: backoff is applied by all stations that transmitted, not just those that caused the collision; there is no inherent priority given to involved devices after backoff; and a device can still collide even if it hears activity on the wire due to propagation delay.