Final answer:
A substance made of eight carbon atoms could be classified as a compound (option C) if it includes other elements, or as a molecule if it refers to an arrangement of carbon atoms alone or with other atoms.
Step-by-step explanation:
If a substance is made of eight carbon atoms, it can be classified as either a compound or a molecule, depending on the specific context of how those carbon atoms are bonded and whether other elements are also present. However, as directly mentioned in the provided information, compounds are substances that consist of two or more elements chemically combined in a fixed proportion. Furthermore, carbon compounds, organic by nature, can be made into molecules composed of many carbon atoms, possibly suggesting that the substance in question with eight carbon atoms is indeed a molecule, and if it's chemically bonded with another element (such as hydrogen) it would also be considered a compound. Given the wording of the question, the most fitting classification is either Compound (C) if the substance contains eight carbon atoms bonded to atoms of another element, or Molecule (D) if it's referencing a particular arrangement of eight carbon atoms alone or in combination with other atoms.