Nitrogen dioxide is a gas that can be generated by emissions from vehicles and factories. It can also be generated by natural sources, such as forest fires, lightning, and microbial activity in soil. The equation for producing nitrogen dioxide is shown below. Which of these provides evidence that a chemical reaction occurs?

D. A brown gas produced

Explanation:

When a chemical reaction occur, there are several changes that takes place such as change in color, odor change, formation of a gas, and formation of a precipitate etcetera.

In the given chemical reaction, two colorless gas (NO and O2) react together and form brown gas (NO2). So, the change of gas from colorless to brown gas is the evidence that shows chemical reaction occurred.

Hence, the correct option is "D. A brown gas produced".