The C - C bond length is maximum in:

We begin by recalling that the length of a C-C bond depends mainly on the type of hybridisation of the carbon atoms involved. The empirical rule is:

$$\text{Greater }\;s\text{-character} \;\Longrightarrow\; \text{shorter bond length}.$$

This rule comes from the basic idea that orbitals with more $$s$$-character are closer to the nucleus, so the bonded electrons are held more tightly, pulling the two nuclei closer together. Let us put this in mathematical form:

$$\text{Bond length} \propto \dfrac{1}{\%\,s\text{-character}}.$$

Now we list the hybridisations and the corresponding $$s$$-character for the carbon-carbon frameworks present in each option:

• In diamond, every carbon is $$sp^3$$ hybridised. From the definition of hybridisation

$$sp^3:\; 1\,s + 3\,p \;\Longrightarrow\; 4\text{ orbitals},$$

the percentage $$s$$-character is

$$\%s = \dfrac{1}{4}\times 100 = 25\%.$$

• In graphite, each carbon is $$sp^2$$ hybridised, so

$$sp^2:\; 1\,s + 2\,p \;\Longrightarrow\; 3\text{ orbitals},$$

hence

$$\%s = \dfrac{1}{3}\times 100 \approx 33.3\%.$$

• In fullerenes C$$_{60}$$ and C$$_{70}$$, the carbon framework is also largely $$sp^2$$ hybridised (with a small curvature‐induced deviation), giving them approximately the same $$s$$-character as graphite, i.e.

$$\%s \approx 33.3\%.$$

Because the bond length varies inversely with the $$s$$-character, we now compare the values:

• Diamond ⇒ $$25\%$$ $$s$$-character ⇒ longest C-C bond.

• Graphite/C$$_{60}$$/C$$_{70}$$ ⇒ $$33.3\%$$ $$s$$-character ⇒ shorter C-C bonds.

Indeed, experimentally measured bond lengths verify this prediction:

$$ \begin{aligned} \text{Diamond}& : \; 1.54 \text{ \AA}\\[4pt] \text{Fullerenes}& : \; 1.46 \text{ \AA (single)} \;\text{and}\; 1.40 \text{ \AA (double)}\\[4pt] \text{Graphite}& : \; 1.42 \text{ \AA} \end{aligned} $$

We see clearly that $$1.54 \text{ \AA}$$ in diamond is the greatest among all the listed structures. Therefore, the C-C bond length is maximum in diamond.

Hence, the correct answer is Option D.