The correct answer is option 1: Automobiles. Automobiles are considered the highest source of air pollution in the world. This is due to the large number of vehicles on the roads emitting harmful gases and particulate matter. The combustion of fossil fuels in car engines releases pollutants such as carbon monoxide, nitrogen oxides, and volatile organic compounds. These pollutants contribute to the formation of smog and harmful health effects. Industries, option 2, also contribute significantly to air pollution, but the sheer number of vehicles worldwide makes them the primary source. Household wastes, option 3, may contribute to localized air pollution, but they do not have as widespread an impact as automobiles. Aircraft, option 4, do release pollutants into the air, but their contribution to global air pollution is comparatively smaller than automobiles.