The gas usually filled inside an electric bulb is:

Electric bulbs, particularly incandescent bulbs, contain a filament that heats up and emits light when electric current passes through it. To prevent the filament from burning too quickly and reacting with oxygen in the air, the bulb is filled with an inert or non-reactive gas. The most commonly used gas for this purpose is nitrogen (N₂).

Nitrogen is chosen because it is chemically inert, meaning it does not easily react with the hot filament, which is usually made of tungsten. By filling the bulb with nitrogen, the oxidation of the filament is prevented, which significantly extends the life of the bulb. In some bulbs, small amounts of other inert gases, such as argon, may be mixed with nitrogen to further reduce filament evaporation and improve efficiency.

Other gases listed in the options are unsuitable. Hydrogen is highly flammable and would pose a serious safety risk. Oxygen would react with the hot tungsten filament, causing it to burn out almost immediately. Carbon dioxide is denser but not commonly used because it does not provide the same level of protection against filament oxidation and can form carbon deposits.

The presence of nitrogen inside electric bulbs is a practical application of physics and chemistry principles, including heat, conductivity, and chemical reactivity. Understanding why inert gases are used helps students connect theoretical knowledge with everyday technology, such as how household bulbs are designed for safety and efficiency.

In summary, the gas usually filled inside an electric bulb is nitrogen, which protects the filament, prevents oxidation, and ensures the bulb’s long-lasting performance. This knowledge is a fundamental part of everyday science and helps explain the physics behind common electrical appliances.