Question

why is copper a pure substance?
because it is an element
because it is a homogenous mixture
because it is a compound

Explanation:

A pure substance has a uniform and definite composition. An element is a pure substance made of only one type of atom. Copper ($Cu$) is an element as it consists of only copper atoms. A homogeneous mixture has a uniform composition but is made of different substances (e.g., salt - water). A compound is a pure substance made of two or more different types of atoms chemically combined (e.g., $H_2O$). Since copper is made of only one type of atom (copper atoms), it is an element and thus a pure substance.

Answer:

Because it is an element.