The danger from radon gas would most likely be greatest in

In Well Insulated Homes

Explanation:

Radon is a radioactive gas which is usually generated in the soil naturally and enters houses via the ground. It is a colourless, odorless, tasteless noble gas.

This gas can enter homes via wall cracks, basement of floors and foundations. The highest level of radon is found in homes that are well insulated, tightly and properly sealed and most likely foundated on uranium-rich soil.