Which substance decays to produce radon-222?

The correct answer is Radium-226, as this isotope is part of the decay chain that ultimately leads to the formation of radon-222. Radium-226 is produced from the decay of uranium-238, but it is specifically the decay of radium-226 itself that results in the release of radon-222. This process is a natural progression in the decay series where radium-226 undergoes alpha decay, resulting in the emission of a radon-222 atom.

Each of the other substances listed also plays a role in different decay chains but does not directly produce radon-222. Uranium-238, while it is a parent isotope that eventually leads to radium-226, does not decay directly to radon-222. Thorium-232 and potassium-40 are isotopes that do not yield radon-222 through their decay processes. Therefore, Radium-226 stands out as the specific isotope that decays to produce radon-222, making it the correct choice.