Which of the following is a primary source of air pollution?

Vehicle emissions are a primary source of air pollution because they release a variety of harmful pollutants into the atmosphere, including nitrogen oxides, carbon monoxide, volatile organic compounds, and particulate matter. These emissions contribute significantly to poor air quality, particularly in urban areas where vehicle traffic is heavy. The combustion of fuels in engines generates these pollutants, which can lead to health issues such as respiratory problems, cardiovascular diseases, and contribute to environmental issues like smog and acid rain.

In contrast, options like tree planting, solar energy, and wind energy are environmentally friendly practices or technologies that help reduce air pollution. Tree planting helps improve air quality by absorbing carbon dioxide and releasing oxygen. Solar and wind energy are renewable energy sources that do not produce air pollutants during operation, unlike fossil fuel-powered vehicles which are significant contributors to air pollution. Therefore, vehicle emissions stand out as a direct cause of air pollution, highlighting their role in environmental degradation and public health concerns.