What conditions require setting the TO DATASET MENU to ALL when OAT is less than 5C?

When operating under conditions where the outside air temperature (OAT) is less than 5°C, it is crucial to ensure safe operations regarding engine performance and ice formation. Setting the TO DATASET MENU to ALL is necessary particularly in two scenarios: when operating on contaminated surfaces and when visible moisture is encountered.

Operating on contaminated surfaces, such as snow or ice, increases the risk of reduced aircraft performance and control. The setting helps ensure that all relevant performance data is considered, allowing for ideal configuration and proper power settings during takeoff.

Likewise, when visible moisture is present, such as rain, fog, or clouds, there's a risk of ice forming on critical aircraft components. This setting is vital to account for the potential impact of ice on engine performance and overall aircraft safety.

Thus, the requirement to set the TO DATASET MENU to ALL is applicable in both cases, which is why the correct answer encompasses both scenarios B and C.