Which of the following conditions is likely to cause the formation of frost?

The formation of frost occurs when temperatures drop below freezing, allowing moisture in the air to condense and freeze on surfaces. Low humidity can also contribute to frost formation because it can lead to clearer skies overnight. When there are fewer clouds to trap heat, temperatures can fall rapidly.

In this scenario, option B—characterized by low humidity and temperatures below freezing—creates ideal conditions for frost. As the temperature drops, available moisture in the air can condense and form frost on surfaces, particularly during calm nights when the heat is radiated away from the ground.

Other options suggest circumstances that would not lead to frost formation. High humidity combined with warm temperatures would keep the air from cooling enough to allow the formation of frost. Clear skies and high temperatures likewise wouldn't result in frost, as warmth prevents the temperature from dropping to the freezing point. Overcast skies and warm temperatures, similarly, retain heat, inhibiting the possibility of frost formation. Therefore, the best condition that aligns with frost formation is found in the option highlighting low humidity and sub-freezing temperatures.