…………….., an inert gas is used to fill weather observation balloons/.

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for an inert gas that is used to fill weather observation balloons.

2. **Understanding Inert Gases**: Inert gases are those that do not readily react with other elements or compounds. They are also known as noble gases.

3. **Consider Common Inert Gases**: The common inert gases include helium, neon, argon, krypton, xenon, and radon. Among these, helium is the most commonly used for filling balloons.

4. **Reason for Using Helium**: Helium is chosen for weather observation balloons because it is lighter than air. This means that when helium is filled in a balloon, the balloon will rise in the atmosphere.

5. **Density Comparison**: The density of helium is significantly less than that of air. This property allows the balloon to float, making it suitable for weather observation.

6. **Conclusion**: Based on the properties of helium and its application, the answer to the question is "Helium".

**Final Answer**: Helium, an inert gas, is used to fill weather observation balloons.

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