______ is the heaviest gas.

Radon gas

The correct answer is option 1. Radon gas

The heaviest gas among the options provided is Radon gas.

Radon (Rn) is a noble gas that is colorless, odorless, and tasteless. It is produced naturally as a decay product of radium and uranium, which are found in varying concentrations in soil, rock, and groundwater. Radon is significantly denser than the other gases listed, making it the heaviest gas among the options provided.