What happens when a transponder malfunctions?

When a transponder malfunctions, it primarily affects the aircraft's ability to be detected on radar displays used by Air Traffic Control. A transponder is responsible for sending out a signal that provides information about the aircraft’s identity, altitude, and other data necessary for air traffic management. If the transponder is not functioning correctly, this can lead to the aircraft becoming invisible to radar, meaning that the radar returns from that aircraft might not be shown or may be incomplete.

This loss of visibility poses a challenge for air traffic controllers, as they rely on transponder signals to manage traffic in the skies safely. Even though the aircraft could still be operating normally and may still fly safely, the lack of radar visibility can complicate navigation and communication with air traffic control, but does not inhibit the aircraft from flying or navigating using other systems.

The other choices suggest more severe or incorrect implications of a transponder malfunction, such as the inability to fly or an immediate need for emergency landing, neither of which are directly related to a transponder issue. Therefore, the correct outcome of a transponder malfunction is that the aircraft becomes invisible to radar.