What might be a physical state of nitrogen dioxide?

Nitrogen dioxide (NO2) is a well-known gaseous compound that is typically recognized by its distinct reddish-brown color. This characteristic color is due to its molecular structure and the way it interacts with light. In the atmosphere, nitrogen dioxide is often found in the form of gas under normal environmental conditions and is associated with air pollution, particularly in urban areas.

While one might encounter nitrogen dioxide as a part of a mixture or under specific conditions that could alter its state, the predominant and observable state of nitrogen dioxide under standard temperature and pressure is indeed that of a reddish-brown gas. This is particularly relevant in discussions regarding air quality, where nitrogen dioxide serves as an important indicator of pollutant levels.

Other options presented do not describe nitrogen dioxide's true state accurately; for example, it does not occur as a colorless gas, clear liquid, or yellow powder under normal conditions. Recognizing the physical state and color of nitrogen dioxide can aid in understanding its impact and behavior in environmental science and industrial applications.