Which of the following is a source of nitrogen dioxide?

Nitrogen dioxide is primarily produced from combustion processes, particularly those that involve fossil fuels. Among the options presented, natural gas is indeed a source of nitrogen dioxide. When natural gas is burned, it can produce various nitrogen oxides, including nitrogen dioxide, due to the high temperatures involved in combustion.

While burning coal also releases nitrogen oxides, including nitrogen dioxide, natural gas combustion is generally associated with cleaner emissions compared to coal. Therefore, nitrogen dioxide emissions from natural gas usage are still relevant when discussing sources, making this option pertinent in the context of the question.

In contrast, vegetation fires produce a range of gases, but they primarily emit carbon monoxide, particulate matter, and other compounds rather than nitrogen dioxide directly. Pesticides, while they can contain nitrogen in various forms, are not a direct source of nitrogen dioxide emissions during their use. Thus, understanding the combustion-related production of nitrogen dioxide leads to recognizing natural gas as a valid answer.