When a combustible substance is burnt, it produces both light and heat, when heat energy that is produced by it is used, called fuel. Usually, fuels are compounds of carbon and hydrogen, thus they combine with oxygen on burning and liberate carbon dioxide and water vapour. Substances such as wood, coal, kerosene, diesel, petrol, LPG, etc., are commonly used as fuels. Fuels may be used at home, in industries and for transportation.
A fuel can exist in which of the following states?

Options

A. Solid only
B. Liquid only
C. Solid and liquid
D. Solid, liquid and gas.

Explanation

**Step-by-Step Solution:**

1. **Understanding Fuels**: A fuel is a substance that produces heat energy when burned. It is typically a compound of carbon and hydrogen.

2. **States of Matter**: Fuels can exist in different states of matter: solid, liquid, and gas.

3. **Examples of Fuels**:
- **Solid Fuels**: An example of a solid fuel is coal.
- **Liquid Fuels**: An example of a liquid fuel is petrol.
- **Gaseous Fuels**: An example of a gaseous fuel is natural gas.

4. **Conclusion**: Since fuels can exist in all three states (solid, liquid, and gas), the correct answer to the question is that a fuel can exist in all three states.

5. **Final Answer**: Therefore, the correct option is "solid, liquid, gas."

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