Final answer:
4) natural sources
Volatile organic compounds (VOCs) mainly originate from natural sources in non-urban environments, such as plants and soil emissions, rather than from automobiles or industrial activity.
Step-by-step explanation:
Except in urban atmospheres, volatile organic compounds (VOCs) come mainly from natural sources. Natural sources of VOCs include emissions from plants, soil, and microorganisms such as bacteria in the guts of termites and ruminant animals.
These compounds typically undergo oxidation in the atmosphere to form carbon monoxide and carbon dioxide. While in urban settings, human-made sources such as automobiles, electric power plants, and other industrial processes are significant contributors of VOCs, in non-urban settings, the natural environment is the predominant source.