Which one of the following substances is not regarded as a true allotrope of carbon in basic chemistry, even though it contains a high proportion of carbon?

Introduction / Context:
Carbon exhibits several different structural forms in which the same element is arranged differently in space. These forms are called allotropes and include diamond, graphite, and newer forms such as graphene and fullerenes. This question tests which of the listed materials is not considered a true allotrope of carbon, even though it is carbon rich and often associated with carbon in everyday life.

Given Data / Assumptions:

• Allotropes of carbon are pure forms with specific crystal structures or arrangements.
• Diamond, graphite, graphene and fullerenes are widely accepted as allotropes.
• Coal is a common carbon containing substance but has a complex composition.
• The question is asked at the level of school chemistry definitions.

Concept / Approach:
Allotropy refers to the existence of an element in more than one form in the same physical state, with different structures and properties. For carbon, classic allotropes include diamond (three dimensional tetrahedral network), graphite (layered hexagonal sheets), graphene (single layer of graphite), and fullerenes (closed cage structures). Coal, although rich in carbon, is not a pure substance but a mixture of various forms of carbon, hydrocarbons, mineral matter, and impurities. Because it does not have a definite, uniform crystal structure or composition, coal is not classified as a true allotrope of carbon.

Step-by-Step Solution:

Verification / Alternative check:
Textbooks on carbon chemistry typically list diamond, graphite and fullerenes as allotropes and may mention graphene as a modern addition. Coal, on the other hand, is discussed in the context of fossil fuels and is described as a complex mixture of carbonaceous material and other substances. It is not given a definite chemical formula or crystal structure. This clear difference in treatment confirms that coal is not an allotrope.

Why Other Options Are Wrong:

• Diamond: A classic allotrope of carbon with a tetrahedral network structure and high hardness.
• Graphite: Another classic allotrope with layered structure and good electrical conductivity along the planes.
• Graphene: A single layer of graphite and accepted as a distinct carbon allotrope in modern discussions.
• Fullerene: A molecular allotrope of carbon with cage like structures such as C60, called buckminsterfullerene.

Common Pitfalls:
Students may incorrectly think that any carbon rich material, including coal or charcoal, is an allotrope. However, only pure forms of carbon with distinct structures qualify. Coal varies widely in composition from one deposit to another and includes many non carbon components. To avoid confusion, memorise the standard list of pure carbon allotropes discussed in your syllabus and treat substances like coal and charcoal as mixtures or impure forms.

Final Answer:
The substance that is not regarded as a true allotrope of carbon is Coal.