Acid rain is primarily a problem of:
a) developed nations.
b) developing nations.
c) primate cities.
d) urban areas.

Well, it could be a problem of all of those, honestly. Even acid rain that develops in one country can go over to another due to the winds and cause problems there.

However, any urban area at this point in time, where there are many people and many gases being produced, will create acid rain, so D is likely to be the answer.