Radon gas is primarily released during the decay of which element?

Radon gas is primarily released during the decay of radium. This occurs as radium undergoes radioactive decay; it transforms into radon as part of its decay chain. Radium-226, a common isotope, decays into radon-222, which is a colorless, odorless gas. This process is particularly significant in environments such as basements and mines where radium is present in geological formations, leading to the accumulation of radon gas.

Uranium and thorium also contribute to radon production indirectly, as they are part of the same decay chains that eventually produce radon, but the most direct source of radon is radium in this context. Osmium does not play a role in radon production, as it is not part of the decay chains that produce this gas. Thus, the association of radon specifically with radium makes it the correct answer.