What gas is used in airships to give them buoyancy?
There are only 2 lifting gasses that are lighter than air and practical for airships;


helium: this is a noble gas which means it's inert. Safest option all around except that US has a monopoly on the production of it. That is also the reason Zeppelins didn't use it because of an embargo the US had at the time against the Germans.
hydrogen: even more buoyant than helium but flammable. It is however easier to get by running a current through water or dropping some metals in an acid.


The third option for providing buoyancy is heated air, this requires a heat source to counteract the loss of heat over time.