The correct answer is: A. brown
Nitric oxide (NO2) is a reddish-brown gas at room temperature. It is a major component of smog and is a toxic air pollutant. NO2 is formed when nitrogen dioxide reacts with oxygen in the air. It is also produced by combustion of fossil fuels, such as coal and oil.
NO2 is a very reactive gas and can react with many other substances, including water, to form nitric acid. Nitric acid is a strong acid that can corrode metals and other materials. It can also cause respiratory problems in humans.
NO2 is a greenhouse gas and contributes to climate change. It is also a precursor to ozone, which is a harmful gas that can cause respiratory problems and other health issues.
NO2 is a regulated air pollutant in many countries. The levels of NO2 in the air are monitored and regulated to protect human health and the environment.
The other options are incorrect because they are not the correct color of NO2 fumes.