Final answer:
Option (A), An element is made up of only one type of atom, while a compound consists of two or more different atoms in a fixed ratio. Molecules of elements consist of only one type of atom, whereas compound molecules contain more than one type.
Step-by-step explanation:
To identify whether a substance is an element, compound, or molecule of an element or compound, we need to consider the composition of the substance at the atomic level. An element is composed of atoms of the same type, each retaining unique properties specific to that element. For example, a piece of iron consists solely of iron atoms. A compound molecule, on the other hand, consists of two or more different types of atoms chemically bonded together in a fixed ratio, like water (H₂O), which is made up of two hydrogen atoms and one oxygen atom.
Moreover, molecules of elements contain only one type of atom, such as an O₂ molecule, which consists of two oxygen atoms bonded together. This is distinct from compound molecules that must contain more than one type of atom. The smallest quantifiable unit that retains the properties of a compound is called a molecule, which can represent either a compound or an element depending on its atomic composition.