The correct answer is C. Carbon.
Carbon is the only element on the periodic table that can exist in four different forms, or allotropes: diamond, graphite, amorphous carbon, and fullerenes.
Diamond is the most well-known allotrope of carbon. It is a hard, transparent solid that is used in jewelry and other applications. Graphite is a soft, black solid that is used in pencils and lubricants. Amorphous carbon is a non-crystalline form of carbon that is found in soot and charcoal. Fullerenes are a group of carbon allotropes that have a cage-like structure.
The different allotropes of carbon have different properties due to the way that the carbon atoms are arranged in each structure. Diamond is made up of carbon atoms that are arranged in a tetrahedral structure. This structure gives diamond its strength and hardness. Graphite is made up of carbon atoms that are arranged in a hexagonal structure. This structure gives graphite its softness and lubricating properties. Amorphous carbon does not have a regular structure, which gives it its variety of properties. Fullerenes are made up of carbon atoms that are arranged in a cage-like structure. This structure gives fullerenes their unique properties, which make them useful in a variety of applications.
The different allotropes of carbon are important because they have different properties that can be used in a variety of applications. Diamond is used in jewelry and other applications that require a hard, transparent material. Graphite is used in pencils and lubricants. Amorphous carbon is found in soot and charcoal. Fullerenes are used in a variety of applications, including electronics, medicine, and materials science.