Solution:
Concept- Carbon has three allotropes: Diamond, graphite and fullerenes , carbon is able to show allotropes due to its property of catenation and pΠ-pΠ bond formation.The allotropes of carbon are-
Diamond: In its structure, each carbon atom is covalently bonded to 4 other carbon atoms, hence carbon is sp3 hybridised in a diamond.
Graphite: In its structure, a carbon atom uses only 3 of its electrons to bond with three other atoms. Hence, carbon is sp2 hybridised in graphite.
Fullerene: Fullerene is one of the allotropes of carbon with the general formula C2n where n=30-48. Most common fullerene is Buckminsterfullerene with formula of C60. It has a shape like a soccer ball. In this, each carbon is sp3 hybridised, thus bonded to three other atoms.
Thus, the microtubules are not the allotropic form of carbon.
Hence, the correct answer is 3.
