Multiple choice :
normally, air pollution episodes are most severe when a) the country breeze is weak
b) a storm system is developing west of the area
c) a cold upper-level low moves over the area
d) a deep high-pressure area stagnates over the area e) a warm front passes through the area

d) a deep high-pressure area stagnates over the area

Explanation: It is usually observed that pollution state gets more worse during the time of high pressure such as during the cold time or winters. During the time of high pressure it becomes difficult for the air pollutants to get mixed with the area and thus they cause the major damage to the environment.

Smog is a example of such phenomena which affects the environment severely and cause breathing problem as well.