Final answer:
The largest source of Carbon Monoxide (CO) is transportation vehicles, primarily due to the combustion of fossil fuels used in cars and trucks. The correct option is 1.
Step-by-step explanation:
The largest source of Carbon Monoxide (CO) is transportation vehicles. CO is a toxic gas released during the combustion of fossil fuels, and it significantly contributes to air pollution, particularly in urban areas.
The other options, like volcanoes, coal burning power plants, and home furnaces, also contribute to CO emissions, but to a lesser extent compared to the emissions from vehicles such as cars, trucks, and other machinery that burn fossil fuels. In addition to outdoor emissions, CO can also come from household sources, which can affect indoor air quality.
Hence, Option 1 is correct.