Which of these is not considered a pure substance?

A) An atom

B) A mixture

C) An element

D) A compound

B and D, and possibly A.

A because a "substance" presumably is made up of a collection of atoms, not just one?

A mixture is basically a mixture of substances - salt and sand eg.

A compound, such as salt, sodium chloride, NaCl is made up of two elements sodium and chlorine.

A mixture is not a pure substance