The correct answer is B.
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 dew point is the temperature at which the air is saturated, or holding as much water vapor as it can.
As the difference between the air temperature and the dew point increases, the air becomes less saturated. This means that there is less water vapor in the air relative to the amount of water vapor that the air could hold at that temperature. Therefore, the relative humidity decreases.
Option A is incorrect because the relative humidity decreases as the difference between the air temperature and the dew point increases.
Option C is incorrect because the relative humidity can never be greater than 100%.
Option D is incorrect because the relative humidity decreases as the difference between the air temperature and the dew point increases.