Which one of the following is not an allotrope of carbon?

Allotropes are different structural forms of the same element in the same physical state [5]. Carbon exhibits several allotropes due to its unique bonding capabilities. Diamond, graphite, and fullerenes (such as Buckminsterfullerene) are well-established allotropes [1]. Graphene is also a recognized allotrope, consisting of a single layer of graphite [4]. While coal contains carbon, it is not considered a pure allotrope of carbon. Instead, coal is a complex mixture of carbon, hydrogen, oxygen, nitrogen, and sulfur, along with mineral matter [3]. Although coal contains amorphous carbon materials, which can be considered allotropic forms, coal itself is a sedimentary rock and a fuel rather than a pure chemical allotrope [1]. Therefore, among the given options, coal is the one that is not a pure allotrope of carbon.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Table 3.1 > p. 40
2. [5] https://en.wikipedia.org/wiki/Allotropes_of_carbon
3. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Allotropes of carbon > p. 61
4. [4] https://www.sciencedirect.com/topics/engineering/carbon-allotrope
5. [3] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > 4.2 VERSATILE NATURE OF CARBON > p. 62