What is a sheet of graphene that has been rolled into a tube called?

A sheet of graphene that has been rolled into a tube is referred to as a carbon nanotube. Carbon nanotubes are cylindrical structures made from graphene and possess unique electrical, thermal, and mechanical properties, making them highly valuable in various nanotechnology applications. Their distinct structure allows for exceptional strength and conductivity, making them ideal for use in materials science, electronics, and nanomedicine.

Graphene itself is a single layer of carbon atoms arranged in a two-dimensional honeycomb lattice, and when this sheet is rolled into a three-dimensional form, it becomes a carbon nanotube. This transformation preserves the properties of graphene but alters its dimensional characteristics, contributing to the unique properties found in carbon nanotubes.

The other options describe different forms of carbon or carbon-related materials. A graphene tube does not represent a standard term in materials science, carbon fiber refers to a material composed of thin strands of carbon atoms woven together (which is quite different in structure and application from carbon nanotubes), and graphite is a bulk form of carbon where layers of graphene are stacked together but not rolled into a tube shape. Thus, the only correct answer that defines a rolled graphene sheet is carbon nanotube.