The answer is helium 0.09 kg/m³.
The wave will move faster in the gas that is having lowest density.
As comparison to other gases-hydrogen, oxygen and radon; helium has the lowest density that is 0.09 kg/m³.
So, waves will move fastest in helium gas as it is having lowest density.
So the answer is helium gas that is waves will move fastest in helium gas.