Which of these is not considered a pure substance?

a)
an atom

b)
a mixture

c)
an element

d)
a compound

A mixture

Explanation:

A mixture is made up of different elements/compounds hence it is not pure

Atoms are the fundamental building blocks. A single atom is so small that it is impossible to see it. It is a pure substance since it cannot be broken down further

Elements are pure substances for example hydrogen, oxygen, sodium copper iron etc.

A compound is also a pure substance since it is made up of two or more elements  

answer; three;