The correct answer is option 3 - Fly ash. An allotrope is a form of an element that has a different atomic arrangement but the same chemical properties. In the case of carbon, it is known to have several allotropes, such as diamond, graphene, and fullerene.

- Diamond is a well-known allotrope of carbon, consisting of a three-dimensional lattice structure. It is one of the hardest known substances and has exceptional optical properties.

- Graphene is another allotrope of carbon, consisting of a single layer of carbon atoms arranged in a two-dimensional hexagonal lattice. It is known for its excellent electrical and thermal conductivity and strength.

- Fullerene is a molecule composed entirely of carbon atoms, arranged in a hollow sphere, ellipsoid, or tube-like structure. It is commonly represented as C60 and has unique properties that have led to its use in various fields.

Option 3 - Fly ash, on the other hand, is not an allotrope of carbon. Fly ash is a byproduct of coal combustion and mainly consists of mineral impurities that are left behind after burning. While it may contain small amounts of carbon, it is not considered an allotrope of carbon itself. Alert - correct answer should be Option 3