The correct answer is: A. At the time when the air temperature is highest.
Relative humidity is the ratio of the amount of water vapor in the air to the amount of water vapor that the air could hold at that temperature. The higher the air temperature, the more water vapor the air can hold. Therefore, relative humidity is usually highest at the time of day when the air temperature is highest.
Option B is incorrect because the middle of the afternoon is not necessarily the time of day when the air temperature is highest. The air temperature can vary depending on the time of year, the location, and the weather conditions.
Option C is incorrect because the air temperature is usually lowest at night. Therefore, relative humidity is usually lowest at night.
Option D is incorrect because just before sunrise is not necessarily the time of day when the air temperature is lowest. The air temperature can vary depending on the time of year, the location, and the weather conditions.
Option E is incorrect because about midnight is not necessarily the time of day when the air temperature is highest. The air temperature can vary depending on the time of year, the location, and the weather conditions.