What indicates that the ice detection system is active when in icing conditions?

The correct choice regarding the ice detection system's activation in icing conditions is indicated by the annunciation of "ICE." This signal is crucial because it informs the flight crew that the aircraft's ice detection sensors have identified the presence of ice accumulation on the airframe.

The "ICE" annunciation prompts the crew to take necessary actions to ensure safe flight operations, such as activating the de-icing systems. This feature is essential for maintaining aircraft performance and safety, as ice buildup can severely affect lift and control.

The illumination of a green light typically indicates normal operations in various systems but does not specifically denote ice detection activity. A warning bell may suggest a different type of alert and is not specifically connected to ice detection. The automatic extension of flaps is unrelated to ice detection and does not signify that the ice detection system is active or has identified ice.