Select the most appropriate option to fill in the blank.
The air quality is so bad ______it becomes difficult to breathe.

Correct option is B

The correct option to fill in the blank is (b) that.
The phrase "so...that" is a correlative conjunction used to show cause and effect. In this sentence, "so bad" indicates the cause, and "that" introduces the effect, which is the difficulty in breathing.
Example: The noise was so loud that we couldn't hear each other.
Why other options are incorrect:
· Option 1: when "When" is used to refer to the time at which something happens, but it doesn't fit in this cause-and-effect structure.
· Option 2: as "As" can mean "because" or "while," but it doesn't correctly link the intensity of the air quality with the difficulty in breathing.
· Option 3: since "Since" is used to indicate time or reason but does not properly convey the cause-and-effect relationship required here.