neon, argon, nitrogen are all possible answers.
Argon. As an inert gas, it makes the filament last longer. Some light bulbs also use Krypton.
Argon gas
argon and xenon i think
In incandescent light bulbs (the regular kind) contain an inert gas, probably Argon. Sometimes there is no gas at all and a vacuum protects the filament. In Florescent bulbs the tubes are filled with...