The correct answer is A. Clouds form as a mass of warm, humid air rises into the atmosphere and cools, condensing moisture into small droplets. This is the correct answer because as warm, moist air rises, it expands and cools, causing the water vapor in the air to condense into tiny water droplets that form clouds. The cooling process leads to the condensation of water vapor, which is the primary mechanism for cloud formation. The other options are incorrect because they do not accurately describe the primary process by which clouds form. Option B describes wind and solar heating affecting moisture, but does not mention the cooling and condensation process. Option C is incorrect because low relative humidity actually inhibits cloud formation. Option D is incorrect because dry air compression does not lead to cloud formation.