Which gas was used to fill the Hindenburg?

Hydrogen, and not by choice. The ship was designed for helium, which does not burn - but the United States held effectively the entire world supply and had banned its export under a 1927 act, partly out of concern about military airships. Germany could not buy any, and flew on hydrogen instead.

The 1937 fire killed 35 of the 97 aboard plus one ground crew, which is a lower proportion than most people assume - it burned so fast that much of it was over in half a minute, and many passengers simply walked out or jumped from low height. What killed the airship industry was not the death toll but that it happened on camera, with newsreel film and a live radio commentator breaking down mid-broadcast.

There is a long-running argument that the hydrogen was not the whole story. The doped fabric skin was treated with compounds including powdered aluminium and iron oxide, chemically comparable to rocket fuel, and a former NASA engineer argued the covering ignited first. Most investigators hold that hydrogen was the primary fuel and the skin contributed, which is a fair reading of an accident that nobody could examine afterwards because it was all gone in thirty-two seconds.