Which gas is colorless, radioactive, and formed by the decay of radium?

Radon is the correct answer because it is a noble gas that is produced through the natural radioactive decay of radium. This process occurs in soil, rock, and groundwater, where uranium decays into radium, and radium subsequently decays into radon. Radon is indeed colorless and odorless, making it difficult to detect without specialized equipment. Its radioactive properties are significant because radon gas can accumulate in buildings, especially in confined areas like basements, posing health risks due to its potential to cause lung cancer upon prolonged exposure.

The other gases listed do not share these specific traits. For example, oxygen is essential for respiration and combustion and is not radioactive, while nitrogen is a major component of the atmosphere and also not radioactive. Argon, a noble gas like radon, is colorless and odorless, but it is not radioactive and does not originate from the decay of radium. Thus, radon stands out as the only gas in the list that meets all the criteria described in the question.