At what temperature should the TO DATA SET MENU on the MCDU be set to ALL?

The correct circumstance for setting the TO DATA SET MENU on the MCDU to ALL occurs when the Outside Air Temperature (OAT) is below 5°C and there is visible moisture. This condition is critical as it addresses the potential for icing conditions, which can significantly affect aircraft performance and safety.

When the temperature is low and moisture is present, there's an increased risk of ice forming on critical surfaces such as wings and engine inlets. By setting the TO DATA SET MENU to ALL under these conditions, the pilot ensures that the aircraft's performance data takes into account the potential for ice contamination, leading to safer takeoff parameters and more accurate performance calculations.

Choosing other temperature ranges, such as above 10°C, between 5-10°C, or merely when the aircraft is on the ground, does not adequately consider the risk of icing and is not the appropriate protocol for the impact of moisture on takeoff performance, which is why they are not the correct choices in this scenario.