The correct answer is A. Clear, calm, dry, winter night.
A radiation inversion is a type of atmospheric inversion that occurs when the temperature of the air increases with height. This can happen when the air is calm and clear, allowing the ground to cool rapidly at night. The cold air near the ground then sinks, trapping the warmer air above it.
Option B is incorrect because moist air is less dense than dry air, so it would tend to rise rather than sink. Option C is incorrect because clouds would block the heat from the ground, preventing the air from cooling rapidly. Option D is incorrect because wind would mix the air, preventing the formation of an inversion. Option E is incorrect because the air is typically moister in the summer than in the winter, so it would be more difficult for an inversion to form.
In addition to the conditions listed above, radiation inversions are also more likely to form in areas with high pressure systems. This is because high pressure systems are associated with sinking air, which can further trap the warm air near the ground.