Concept:A substance made up of only one kind of atom is called an element.
Explanation:An element is a pure substance in which all atoms are of the same type.
For example, iron contains only iron atoms, and oxygen contains only oxygen atoms.
A compound is formed when two or more different elements are chemically combined.
A mixture is formed when two or more substances are combined physically, without any chemical reaction.
An ion is an atom or a group of atoms that carries a positive or negative electric charge.
Therefore, the substance described in the question is an element.
Answer:B. an element