The greatest source of indoor air pollution is
photochemical smog
methyl chloride
asbestos insulation
cigarette smoke

The correct answer would be option D, Cigarette Smoke.

Explanation:

Air Pollution is simply the pollution in the Air. Air pollution consists of gases or solid particles. Air pollution can be indoor and out door. Outdoor pollution is caused by the chemical emissions, factory gases emission, dust, car emissions, etc. The indoor air pollution would be considered the pollution that is caused by the indoor sources, such as fuel burning, smoke generation, etc. So the source of indoor air pollution which is considered the greatest is Cigarette Smoke, as large amount of people smoke inside the facilities, increasing the air pollution.

a bird that nests only in a particular tree species.