Final answer:
At 100% Relative Humidity, the air is fully saturated with moisture and has reached its maximum water vapor capacity, often resulting in fog or dew formation.
Step-by-step explanation:
When the Relative Humidity (RH) reaches 100%, the condition of the air is that it is fully saturated with moisture (Option B). This means the air has reached its maximum moisture-holding capacity, which can lead to the formation of fog or dew. The dew point is the temperature at which the relative humidity is 100% and is the temperature at which water starts to condense out of the air. A higher relative humidity can make temperatures feel warmer because the body's sweat does not evaporate as quickly, but it is not directly related to the air being at its highest temperature nor does it indicate a heatwave.