Air pressure is usually highest when the air is
a.) cool and humid
b.) cool and dry
c.) warm and humid
d.)warm and dry

The answer is B. cool and dry.

High pressure is usually associated with Fair weather, in other words clear skies. When the air cools, it gets denser and it is pushed down to the ground. Now the pressure increases because as the air is pushed down, it leaves more room for more air to fill in that space. As the warm air rises up to fill in that space, it evaporates the moisture in the air that would have made clouds. 

B.) cool and dry

Explanation:

The cold and dry air is main cause of the high-pressure areas as the pressure tends to be highest when the air is dense and the ground is also dry as the warm and the moisture conditions present on the air can bring about a changes in the air pressure of the place as the air mass will tend to move for the place of the low and places of the high pressure. Thus development construction pattern of the air and which ultimately leads to the duration of the front and the development of the cyclonic storm. For that air to have a high pressure the winds need to be calm and the dry so that the cold air can create pressure zone and attract the low pressure or the warm air towards it.

It is the second answer