The correct answer is D. Carborundum.
An allotrope is a form of an element that has a different structure than other forms of that element. Carbon has many allotropes, including soot, graphite, diamond, and fullerenes.
Soot is a black powder that is produced when carbon-containing materials are burned incompletely. It is made up of tiny carbon particles that are bonded together in a disordered way.
Graphite is a soft, black mineral that is made up of carbon atoms that are arranged in a hexagonal lattice. It is a good conductor of electricity and heat.
Diamond is a hard, colorless mineral that is made up of carbon atoms that are arranged in a tetrahedral lattice. It is the hardest natural substance on Earth.
Carborundum is a hard, abrasive material that is made up of silicon carbide. It is not an allotrope of carbon because it does not contain carbon atoms.