Answer
Propane
Work Step by Step
Based on the molar mass will be propane.
$M_{propane}=44.1\frac{kg}{kmol}$
$M_{methane}=16\frac{kg}{kmol}$
$M_{air}=29\frac{kg}{kmol}$
As you can see propane is heavier than air and will stay near the floor but methane is lighter and it will rise and go out.
