The C–C bond length is maximum in:

Strategy: Compare C–C bond lengths across allotropic forms of carbon.

Step 1: Bond Character and Length Bond length is inversely related to bond order and hybridisation character:

• Diamond: $sp^3$ hybridised, pure single ($\sigma$) C–C bonds. Bond length ≈ 154 pm (longest).
• Graphite: $sp^2$ hybridised, bonds have partial double-bond character (~1.33 bond order). Bond length ≈ 141.5 pm.
• $\ce{C60}$ (Fullerene): $sp^2$ hybridised, two types of bonds (~138–145 pm), shorter than graphite overall.
• $\ce{C70}$: Similar to $\ce{C60}$, bonds range ~137–146 pm.

Step 2: Conclusion Among these allotropes, Diamond has the longest C–C bond length due to pure $sp^3$ single bonds.

$\boxed{\text{Answer: (D) Diamond}}$