Which of these are gas balloons filled with?

That question doesn't make sense.

I got the answer correct. Didn't find it confusing. But then again, I was born in a different galaxy.

Why are people objecting to this question? This question is about gas balloons -- not hot-air balloons, which came later. Propane, whether in liquid or vapor form, is heavier than air. Oxygen is heavier than air. Therefore neither will rise into the air. A balloon filled with cold air will also not rise. That leaves only one option: a gas lighter than air.

So confusing!