Graphene is

Graphene is a single layer of carbon atoms arranged in a two-dimensional hexagonal lattice, with each atom bonded to three neighbours through sp2 bonds. That makes it a nanostructure of carbon, so option B is correct. Its thickness is one atom, and it was first isolated in 2004 by Geim and Novoselov, who received the Nobel Prize in Physics in 2010.

Why the others fail:

• A (alloy): An alloy is a mixture of a metal with another element or metal. Graphene is pure carbon, so it is not an alloy.
• C (isotope): Isotopes are atoms of the same element differing only in neutron number. Graphene is a structural arrangement of many carbon atoms, not a variant of a single atom.
• D (none of the above): Wrong, because B is correct.

The core concept is allotropy. Diamond, graphite, fullerenes, carbon nanotubes and graphene are all forms of carbon that differ in bonding and structure. For Mains GS-III (science and technology, nanotechnology), graphene is useful for its strength, conductivity and possible uses in electronics, energy storage and composites. The fact has not changed since the exam.

Trap: Aspirants sometimes confuse graphene with graphite. Graphite is a three-dimensional stack of many graphene layers, while graphene is a single layer.