Identify a substance that is NOT an allotrope of carbon.

The correct option is Phosphorus.

Allotropes are different structural forms of the same chemical element in the same physical state, possessing distinct physical properties while sharing similar chemical behavior.

Let us evaluate each option:

1. Graphite: Graphite is a well-known allotrope of carbon in which carbon atoms are bonded in planar hexagonal sheets.

2. Buckminsterfullerene: Buckminsterfullerene (C₆₀) is a soccer-ball-shaped fullerene allotrope composed entirely of carbon atoms.

3. Diamond: Diamond is a crystalline allotrope of carbon in which each carbon atom is tetrahedrally bonded to four other carbon atoms in a 3D network.

4. Phosphorus: Phosphorus is an entirely separate non-metallic chemical element (symbol P, atomic number 15). It has its own distinct allotropes (such as white, red, and black phosphorus) and is not an allotrope of carbon.

Therefore, Phosphorus is the substance that is NOT an allotrope of carbon.