_______ is the hardest known natural substance.

Options

A. Diamond
B. Fullerene
C. Graphite
D. All of these

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for the hardest known natural substance.

2. **Consider the Options**: The options provided are diamond, fullerene, and graphite. All of these are allotropes of carbon.

3. **Understand Allotropes of Carbon**: Allotropes are different forms of the same element. In this case, we have:
- **Diamond**: Each carbon atom is sp³ hybridized and forms a strong three-dimensional network.
- **Graphite**: Carbon atoms are arranged in layers, with weaker bonds between the layers, making it softer.
- **Fullerene**: A molecular form of carbon that is not as hard as diamond.

4. **Analyze the Structure of Diamond**:
- Diamond has a tetrahedral structure where each carbon atom is bonded to four other carbon atoms through strong covalent bonds.
- This arrangement creates a very strong and rigid three-dimensional structure.

5. **Conclude the Hardness**: Due to its strong covalent bonding and three-dimensional structure, diamond is known to be the hardest natural substance.

6. **Final Answer**: Therefore, the hardest known natural substance is **diamond**.

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