Explanation

(a)
The $C - C$ bond length in$\ sp^{3}$ hybridisation is greater than $sp^{2}$ hybridisation due to large size of $p$-orbitals.
In diamond $sp^{3}$ hybridisation is present while in graphite, naphthalene and fullerene $sp^{2}$ hybridisation is present therefore the $C - C$ bond length is maximum in diamond.