What is the role of the earphone at the listener's station?

• A. To route the audio signal to other stations.
• B. To monitor the line with a loudspeaker.
• C. To amplify the received signal electronically.
• D. To convert the received electrical signal back into audible sound.

Answer: (D)

The earphone at the listener’s station is the receiving transducer that converts the electrical signal on the line back into audible sound for the listener. In a sound-powered system there’s no external power or electronic amplification—the signal energy itself is used to drive the earphone so you hear the message directly. This is why it’s the correct function: it is designed to turn the electrical signal back into sound you can listen to.

The other ideas don’t fit because routing to other stations is handled by distribution or switching beyond the listener’s own device, a loudspeaker would be used for supervisor monitoring rather than the personal listening station, and amplification isn’t part of the listener’s earphone in a power-free system.