The correct answer is option 3: 2 only.

Diamond, which is option 1, is not a good conductor of electricity. This is because diamond has a tightly bonded structure with each carbon atom forming covalent bonds with four other carbon atoms, creating a tetrahedral pattern. This strong bonding makes it difficult for electricity to flow through diamond, hence making it a poor conductor.

Graphite, which is option 2, is a good conductor of electricity. In graphite, each carbon atom is covalently bonded to three other carbon atoms, forming sheets of hexagonal rings. These sheets are then stacked on top of each other, with weak intermolecular forces between them. These weak forces allow the electrons to move easily between the layers, enabling the conduction of electricity.

Fullerene, which is option 3, is not a good conductor of electricity. Fullerene is a type of carbon molecule where carbon atoms are arranged in hollow cages or tubes. While fullerene can conduct electricity to some extent, it is not as efficient as graphite.

In summary, graphite is the only carbon allotrope among the options listed that is a good conductor of electricity.