Which term refers to the various forms of a single chemical element?

The correct term that refers to the various forms of a single chemical element is allotropes. Allotropes are different structural forms of an element in the same physical state. For instance, carbon has several allotropes including graphite, diamond, and graphene, all of which have distinct physical properties despite being composed of the same type of atoms.

Isotopes, on the other hand, relate to variations of an element that have the same number of protons but different numbers of neutrons. This means that isotopes pertain to different atomic masses of an element rather than distinct structural forms.

Compounds are substances formed when two or more different elements bond together chemically. For example, H2O is a compound made up of hydrogen and oxygen.

Elements refer to pure substances that consist of only one type of atom. While this choice is fundamental to chemistry, it doesn't articulate the concept of different structural forms of the same element.

Therefore, the terms related to forms or structures of a single chemical element are best summarized by the concept of allotropes.