Which underground gas is commonly referred to as "laughing gas"?

The term "laughing gas" commonly refers to nitrous oxide, which is not listed in your choices. However, if the intention is to associate the term with one of the options provided, it's important to clarify why none of the options correctly match this designation since nitrous oxide is different from nitrogen dioxide, which is toxic and can lead to respiratory issues.

Nitrous oxide (N2O) is known for its euphoric and anesthetic effects, leading to the nickname "laughing gas." Nitrogen dioxide (NO2), while related in name, is a dangerous pollutant that can cause significant harm to health and the environment. It is a byproduct of combustion and can lead to various respiratory problems, making it quite different in functionality and effects compared to nitrous oxide.

Thus, while nitrogen dioxide may resemble "laughing gas" in its naming convention, it does not share the benign and even pleasurable characteristics associated with nitrous oxide. Consequently, when considering the correct context of "laughing gas," none of the provided choices align with the well-known definition.