The correct answer is option 1: Coal. Coal is not an allotrope of carbon. An allotrope is a different form or structure of an element, in this case, carbon. Allotropes have different physical and chemical properties.

Option 2: Diamond is an allotrope of carbon. It is a naturally occurring form of carbon that is known for its hardness and clarity. It forms under high temperature and pressure and is made up of a three-dimensional arrangement of carbon atoms.

Option 3: Graphite is another allotrope of carbon. It is a soft, black, and slippery substance. It consists of layers of carbon atoms arranged in a hexagonal lattice structure.

Option 4: Graphene is also an allotrope of carbon. It is a single layer of graphite and is one of the thinnest and strongest materials known. Graphene has unique electronic properties and is being studied for a wide range of applications.

In summary, while coal is made up of carbon, it is not considered an allotrope of carbon. Allotropes of carbon include diamond, graphite, and graphene.