Final answer:
The lighter-than-air gas used in the Hindenburg was hydrogen, which burned in air and caused the disaster. Although helium is lighter than air and safer to use, it was not used in the Hindenburg due to cost and export restrictions.
Step-by-step explanation:
The lighter-than-air gas used in the Hindenburg was a) Hydrogen.
Hydrogen is a combustible gas that burns in air. It reacted with oxygen during the Hindenburg disaster, resulting in a fireball that destroyed the airship. Although hydrogen is lighter than air and has lifting power, it is a dangerous choice due to its flammability. Helium gas, on the other hand, is also lighter than air and safer to use because it does not burn. However, helium was not used in the Hindenburg due to its higher cost and the United States' ban on helium exports to Germany at the time.