Final answer:
The correct answer is C) Air is saturated when air reaches its dew point, indicating that the relative humidity is 100%, and the air cannot absorb additional water vapor.
Step-by-step explanation:
When air has reached its dew point, the correct answer is C) Air is saturated. At the dew point temperature, the relative humidity is 100%, and additional water vapor can no longer be absorbed into the air. The air's capacity to hold water vapor is a common but technically inaccurate way to describe saturation - it is actually determined by vapor pressure, which is directly related to temperature. High relative humidity in the evening, as the air temperature declines, can lead to the air reaching its dew point. When the dew point is reached, any further cooling may cause condensation, producing phenomena like fog or dew, and this condensation process releases heat which tends to prevent the temperature from dropping further.