Final answer:
The closer the wet-bulb and dry-bulb temperatures are to each other, the higher the relative humidity of the air. This is because the air is near saturation and cannot hold much additional water vapor.
Step-by-step explanation:
When the wet-bulb and dry-bulb temperatures are close to each other, it indicates that the air is near saturation and cannot hold much more water vapor, meaning that the relative humidity is high. Therefore, the correct answer to the question is A) Relative humidity.
Relative humidity measures how much water vapor is present in the air compared to the maximum amount of water vapor the air can hold at a given temperature. When relative humidity approaches 100%, it means the air's capacity to hold water vapor is almost reached, and additional cooling will lead to condensation, which is the point when the dew point temperature is reached.
The closer the wet-bulb and dry-bulb temperatures are to each other, the higher the Dew point. The dew point is the temperature at which 100% relative humidity would occur. When the wet-bulb and dry-bulb temperatures are close, it means that the air is near saturation and the dew point is high.
Therefore, the correct answer to the question is A) Relative humidity.