The gas used in gas thermometer is 

Options

A. He
B. `O_2`
C. Xe
D. N e

Answer

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks for the gas that is commonly used in gas thermometers.

2. **Identifying the Options:**
The options provided are:
- Helium
- Oxygen
- Neon
- Argon

3. **Analyzing the Properties of Each Gas:**
- **Helium:** It is a noble gas, known for being non-reactive and has a low boiling point, making it suitable for temperature measurements.
- **Oxygen:** It is reactive and can participate in combustion, which makes it unsuitable for use in thermometers.
- **Neon:** Another noble gas, but it is less commonly used than helium in gas thermometers.
- **Argon:** Also a noble gas, but helium is preferred due to its lower density and better thermal conductivity.

4. **Conclusion:**
Based on the properties of these gases, helium is the most suitable gas for use in gas thermometers due to its non-reactive nature and physical properties.

5. **Final Answer:**
The gas used in gas thermometers is **Helium**.

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