Question

a pure substance is: composed of only one type of atom or molecule. composed of two or more regions with different compositions. composed of two or more different types of atoms or molecules that has constant composition.

Explanation:

A pure substance has a fixed and definite composition. It is either an element (made of one - type of atom) or a compound (made of one - type of molecule). Options with multiple regions of different compositions or multiple types of atoms/molecules with constant composition describe mixtures or compounds in a more complex context, not pure substances in the simplest sense.

Answer:

composed of only one type of atom or molecule.