The correct answer is A. nitrogen.
Electric bulbs typically contain chemically inert gases such as nitrogen to prevent the filament from burning up. Nitrogen is a non-reactive gas that does not react with the filament, even at high temperatures. This helps to extend the life of the bulb.
Chlorine is a reactive gas that can react with the filament, causing it to burn up more quickly. Oxygen is also a reactive gas that can react with the filament, but it is less reactive than chlorine. Hydrogen is a flammable gas that can cause the bulb to explode.
Therefore, the best gas to use in an electric bulb is nitrogen. It is non-reactive, does not react with the filament, and does not cause the bulb to explode.