In an ordinary electric filament bulb, which gas is usually filled inside the glass bulb to protect the filament and improve its performance?

Introduction / Context:
Electric filament bulbs produce light when an electric current heats a thin filament until it glows. The environment around this filament has a major influence on its life and efficiency. To prevent the filament from burning out quickly, the air inside the bulb is carefully controlled. This question focuses on which gas is usually filled inside such bulbs in basic school level descriptions.

Given Data / Assumptions:
- The device is an ordinary electric filament bulb, not a modern LED lamp.
- The question asks which gas is usually filled inside the bulb.
- Options include hydrogen, oxygen, nitrogen, and carbon dioxide.
- We assume the traditional teaching that mentions nitrogen as the filling gas where inert gases such as argon are not listed.

Concept / Approach:
If normal air were present inside a filament bulb, the hot filament would quickly react with oxygen and burn out. Therefore, the bulb is either evacuated or filled with an inert or non reactive gas. In many basic general science questions, nitrogen is given as the gas used inside bulbs because it is relatively inert at room temperature and helps reduce evaporation of the filament. In more detailed physics, argon or a mixture of inert gases is often mentioned. However, among the options provided here, nitrogen is the only gas that fits the role of a protective, relatively non reactive filling gas, while oxygen would speed burning and hydrogen or carbon dioxide are not standard choices for filament bulbs.

Step-by-Step Solution:
Step 1: Recognise that the main problem is preventing the hot filament from reacting with gases inside the bulb. Step 2: Understand that gases which support combustion, such as oxygen, would damage the filament. Step 3: Recall from school physics that nitrogen or inert gases are used to fill bulbs and protect the filament. Step 4: Among the given options, identify nitrogen as the only gas commonly used for this purpose in simple descriptions. Step 5: Choose nitrogen as the correct answer.

Verification / Alternative check:
Textbooks at school level often state that the bulb is filled with an inert gas such as nitrogen or argon. Question banks and competitive exam guides usually offer nitrogen as the answer when argon is not in the option list. Since inert gases are preferred because they do not readily combine with the hot filament, nitrogen fits this requirement better than the other listed gases. This confirms that nitrogen is the best choice from the given options.

Why Other Options Are Wrong:
- Hydrogen is highly flammable and would make the bulb dangerous; it is not used as a filling gas in ordinary bulbs.
- Oxygen strongly supports combustion and would cause the filament to oxidise and burn quickly.
- Carbon dioxide, while less reactive than oxygen, is not the standard gas used in filament bulbs and is not usually mentioned in school level descriptions.

Common Pitfalls:
Learners may remember that air is removed from the bulb and assume that the bulb is a complete vacuum, or they may incorrectly pick oxygen because they associate it with burning and light. It is important to recall that the purpose of the gas is to protect, not destroy, the filament. Remembering that nitrogen and argon are commonly used gases in bulbs can help avoid confusion in similar questions.

Final Answer:
The gas usually filled in an electric filament bulb is Nitrogen.