What type of gas is used to fill aircraft balloons to prevent explosions?

Helium is used to fill aircraft balloons primarily because it is non-flammable and much safer to handle than other gases such as hydrogen. The use of helium reduces the risk of explosions, which can occur with hydrogen due to its flammable nature. Helium, being an inert gas, does not support combustion, making it the preferred choice for applications where safety is a paramount concern, such as in the aviation industry.

Additionally, helium is lighter than air, which allows the balloons to rise while providing a level of safety that ensures they do not pose a fire hazard. This characteristic is particularly vital in aviation, where the implications of a fire can be catastrophic. The other gases listed do not share these important safety features, which is why helium is the optimal choice for this purpose.