Hydrogen gas, by far, has the lowest density out of those 4 choices.
You might think of it in terms of "what will float in what". Gold sinks in water, so it must be more dense. Gasoline float on water, so it is less dense. Hydrogen gas will definitely "float" in gasoline, so it is the lease dense.
Or you could simply look up each items density, they are pretty common substances, the values will not be hard to find.