The most important reason for air pollution is/are factories.
As the actual subject of the above sentence is factories: a plural noun
I wonder if I should write 'is' or 'are' as the main verb!
I think the verb 'is' will be appropriate here.
It would be much more natural if it started "The main cause of ..."
NOT A TEACHER
Or it could also start, "The greatest/largest source of air pollution is ...".
So I should write the main cause of air pollution is fctories.
or the great source of airpollution is factories.