Answer is: (4) different molecular structures and different properties.
Different forms of the same element that have different properties because of different atom arrangements are called allotropes.
Carbon has many allotropes, but two most important are graphite and diamomd.
Graphite has sp2 and diamond has sp3 hybridization of carbon atoms, because of that graphite conduct electricity and diamond not.
In diamond carbon atoms are arranged in the face centered cubic crystal structure called a diamond lattice.
Diamond has very strong covalent bonds between carbon atoms and because of that it has the highest hardness and thermal conductivity of any bulk substance.
