What condition necessitates turning the engine anti-ice system ON in flight?

The correct answer is that the engine anti-ice system should be turned ON when visible moisture is detected. This is crucial because the presence of visible moisture, such as rain, snow, or clouds, in conjunction with lower outside air temperatures can create conditions conducive to ice accumulation on engine components. Ice formation can disrupt engine performance and lead to potential safety hazards during flight.

While climbing provides certain altitude changes and may expose the aircraft to different temperature layers, it does not directly indicate the need for anti-ice systems. Similarly, temperatures at or above 10°C generally do not pose a significant risk for ice formation, and conditions below 0°C typically suggest the need for caution, but it is the combination of moisture presence that really drives the necessity for activating the anti-ice system. Therefore, it's essential to monitor moisture conditions closely to mitigate any risks of ice during flight.