When can wing anti-ice be used without restrictions related to temperature?

The correct choice indicates that wing anti-ice can be used without restrictions when the Static Air Temperature (SAT) is -40°C or below. This range is significant because at such low temperatures, the risk of ice formation is substantially decreased, making the use of anti-ice systems less impactful or necessary.

Ice can only form under conditions when moisture is present at temperatures above approximately -40°C. Therefore, if the air temperature is at or below this threshold, the likelihood of ice accumulation on the wings is minimal regardless of moisture conditions, hence users can activate wing anti-ice systems without worrying about potential adverse effects or limitations due to temperature.

The other options do not represent conditions that provide the same unrestricted use of wing anti-ice. For instance, wing anti-ice may be limited or avoided at temperatures above 10°C because ice is unlikely to form in warmer conditions. Likewise, cruising at high altitudes does not inherently indicate a guarantee that it's safe to use wing anti-ice, as various factors, including temperature, moisture, and air pressure, need to be considered.