Under which condition must wing anti-ice be on for takeoff?

The correct answer indicates that wing anti-ice must be activated when visible moisture is reported below 400 feet above ground level (AGL). This is crucial for preventing ice accumulation on the wings, which can significantly affect aircraft performance by altering lift characteristics and increasing drag. When flying in conditions where visible moisture is present, particularly in temperatures near or below freezing, the risk of ice formation increases.

The requirement to have wing anti-ice on under these conditions is aligned with safety protocols designed to ensure optimal performance and handling during critical phases of flight such as takeoff. In scenarios with visible moisture and cold temperatures, ice can form quickly and unintentionally if anti-ice systems are not engaged, leading to potential loss of control or performance issues.

In contrast, while factors like airspeed, runway conditions, and outside air temperature (OAT) are important, they do not specifically mandate the use of wing anti-ice on takeoff in the same context as the presence of visible moisture. Thus, activating the system under the condition stated is vital for safe operation, ensuring that any ice formation is countered effectively.