When must the wing anti-ice be activated according to the ice detection system?

The correct choice indicates that the wing anti-ice must be activated when ice is annunciated by the ice detection system. This is crucial for safe flight operations because the ice detection system is designed to monitor conditions that could lead to ice accumulation on the wings. Ice on the wings can significantly impact aircraft performance, including reducing lift and increasing drag.

Activation of wing anti-ice upon detection ensures that any potential ice build-up is mitigated quickly, maintaining the aircraft's aerodynamic efficiency and safety. This response is a direct and immediate reaction to the system's alerts, ensuring that de-icing measures are taken precisely when necessary.

The other potential choices relate to temperature and flight conditions, which may suggest situations where ice could form. However, they do not trigger the anti-ice system as directly as the annunciation of ice itself does. Temperature and certain airspeeds might inform a pilot's awareness of potential icing conditions, but the activation of the system is specifically directed by the ice detection alerts to ensure timely and effective response to ice accumulation risks.