Final answer:
Different structural forms of the same element are called allotropes, which have the same physical state but different molecular structures and properties. Option c is correct.
Step-by-step explanation:
Different structural forms of the same element are called allotropes. Allotropes are various forms that an element can take on, which exist in the same physical state but with different molecular structures. This can occur due to variations in how atoms are bonded together in a substance. A common example of allotropes can be found in carbon, which exists as graphite, diamond, and fullerene, each with distinct properties despite being made up of the same carbon atoms. For example, carbon can exist as diamond, graphite, and fullerenes, which are all different allotropes of carbon.
Metals and transition elements are not the correct answers, as these terms relate to types of elements rather than different structural forms of a single element. Diatomic molecules are a form of molecules consisting of two atoms, which is also not applicable in this context. Metallic bonding is the force that holds metal atoms together in a metallic solid and is not a term that describes different structural forms.