Just as ’major’ describes a military rank and ‘principal’ a school one, they apply to describe the scale of the importance of a matter. ‘Principal’ means the most important, whatever it is – ingredients in a soup or rules to follow. It can be described with adjectives: ‘main’ or ‘basic’. Example: Three principal values are the basis of friendship: trust, honesty and common interests.
Whereas ‘major’ implies more influence, power and importance. It can be described with adjectives: ‘leading’, ‘significant’ and ‘great’. Example: Deforestation, fossil fuels combustion and livestock are reported to be the major causes of greenhouse gases.

In your sentence: Also, looking at the air pollution markers, let’s start looking downtown, probably near a (5) Correct answer: C,MAJOR intersection’ – the right answer is ‘major‘ because it represents its leading role or great importance in traffic control. Large dimensions of the intersection may factor in.