What are the requirements for anti-ice in flight when there is visible moisture?

The choice indicating that anti-ice is required when the Total Air Temperature (TAT) is between 10°C to -39°C and there is visible moisture reflects established aviation guidelines for operating in conditions that can lead to ice formation. This range is critical because it encompasses temperatures where the presence of moisture can result in the formation of ice on aircraft surfaces.

When flying within this temperature bracket, visible moisture presents a significant risk for ice accumulation, which can impair aircraft performance and safety. Therefore, engaging anti-ice measures becomes essential to mitigate this risk effectively.

In contrast, other choices either misinterpret the conditions under which anti-ice should be activated or oversimplify the requirements. Thus, the provided correct answer effectively highlights the specific temperature range and moisture condition that necessitates anti-ice procedures, ensuring safe flight operations within these parameters.