Concept:The difference between wet and dry bulb thermometer readings shows how much moisture is in the air.
Explanation:A wet bulb thermometer cools by evaporation.
When the air contains much moisture, evaporation is slow.
So the wet bulb reading remains close to the dry bulb reading.
This gives a small difference between the two thermometers.
A small difference means the air is nearly saturated with water vapour.
Therefore, the relative humidity is high.
Answer:D. high relative humidity