Which of the following is a negative externality?

A negative externality occurs when an economic activity imposes costs on third parties who are not involved in the activity itself. In this case, pollution from vehicles impacting air quality exemplifies a negative externality because it creates detrimental effects for members of the community who suffer from reduced air quality. These individuals may experience health issues, decreased quality of life, and other harmful consequences as a result of the pollution without receiving any compensation from those responsible for the vehicle emissions.

The other options presented are examples of positive contributions to the environment and society. Planting trees in an urban area generally improves air quality, provides shade, enhances beauty, and supports biodiversity. Community gardens foster local biodiversity and enhance community interactions, while animal husbandry can contribute positively to ecosystem balance, especially when done sustainably. None of these activities create the harmful costs associated with negative externalities. Thus, the pollution from vehicles is correctly identified as a negative externality due to its harmful impact on the community.