The longest C−C bond is present in

Options

A. Graphite
B. Diamond
C. `C_60`
D. `C_70`

Explanation

To determine which allotrope of carbon has the longest C−C bond, we will analyze the bond lengths of various carbon allotropes: graphite, diamond, C60 (Buckminsterfullerene), and C70.

### Step-by-Step Solution:

1. **Identify the Allotropes of Carbon**:
The question provides four allotropes of carbon: graphite, diamond, C60, and C70.

2. **Examine the Structure and Hybridization**:
- **Graphite**:
- Structure: Sheet-like 2D structure.
- Hybridization: sp².
- C−C bond length: 141.5 picometers.
- **Diamond**:
- Structure: Tetrahedral geometry with a cage-like structure.
- Hybridization: sp³.
- C−C bond length: 154 picometers.
- **C60 (Buckminsterfullerene)**:
- Structure: Spherical structure made of hexagons and pentagons.
- Hybridization: sp².
- C−C bond length: approximately 143 picometers.
- **C70**:
- Structure: Elongated spherical structure.
- Hybridization: sp².
- C−C bond length: approximately 138 picometers.

3. **Compare the C−C Bond Lengths**:
- Graphite: 141.5 pm
- Diamond: 154 pm
- C60: 143 pm
- C70: 138 pm

4. **Identify the Longest Bond**:
From the bond lengths analyzed, we see that diamond has the longest C−C bond length at 154 picometers.

5. **Conclusion**:
Therefore, the longest C−C bond is present in diamond.

### Final Answer:
The longest C−C bond is present in **diamond** (option B).