Select the most appropriate option to fill in the blank.

The air quality is so bad ______it becomes difficult to breathe.

• The correct conjunction to complete the sentence "The air quality is so bad ______ it becomes difficult to breathe" is "that".
• The conjunction "that" is used to introduce a clause that is the result or the consequence of the main clause.
• The other options: "when", "as", and "since" do not fit the context as appropriately.

​Therefore, the correct answer is 'Option 2'.

Complete Sentence: The air quality is so bad that it becomes difficult to breathe.

• When (जब): is used to refer to the time at which something happens.
• As (जैसे): is often used to indicate similarity or manner.
• Since (चूंकि/जब से): is used to indicate a point in time or reason.