What is the name of the carbon allotrope used in pencil leads?

The carbon allotrope used in pencil leads is graphite.

Graphite consists of carbon atoms arranged in layered sheets. Within each sheet, atoms form strong hexagonal networks, while the layers are held together more weakly. When a pencil moves across paper, tiny graphite particles shear away and remain on the surface, creating the familiar gray mark.

Modern pencil “lead” is not actually lead metal. Most cores are made from powdered graphite mixed with clay and a binder. More graphite generally produces a softer, darker mark, while more clay produces a harder, lighter grade. The grading system, including labels such as HB and 2B, reflects this balance and manufacturing process.

The other choices describe different carbon forms or structures. Diamond has a rigid three-dimensional atomic network and is extremely hard; graphene is a single sheet of graphite; and fullerenes are closed carbon molecules. Graphite is also electrically conductive, because some electrons can move within its layers, unlike diamond.