Which is the least probable source of atmospheric pollution in heavily populated cities?

Question

(1).human activities
(2).industrial plants
(3).natural processes
(4).automobile traffic

Answer 1

It depends on your definition of terms. The term natural processes used to be countered by supernatural processes. From the current attitude I assume that actions of people are not considered natural. Since Cities are where there is a great density of people natural processes are lower on the list of sources of pollution than other locations.
Industrial plants are usually sited some distance from heavily populated areas.
One of these two is probably the answer being sought by the creator of this question.
I would choose the industrial plant because very few if any exist in heavily populated cities.