What is a key source of nitrogen oxides?

Nitrogen oxides are primarily formed at high temperatures, particularly in processes such as combustion and industrial activities. When fossil fuels are burned at elevated temperatures—like in car engines, power plants, or industrial furnaces—the nitrogen in the air reacts with oxygen to create nitrogen oxides (NO and NO2). This reaction is facilitated by the high energy present during combustion, which provides the activation energy needed for the reaction to take place.

While other options may contribute to the overall production of nitrogen compounds, they do not primarily produce nitrogen oxides. For example, natural decay processes and photosynthesis don't involve the high-energy conditions necessary for forming nitrogen oxides. Incomplete combustion of fuels can lead to carbon monoxide and other pollutants but is not the most significant source of nitrogen oxides compared to high-temperature industrial processes. Thus, the correct answer highlights the critical role of elevated temperatures in generating nitrogen oxides.