Which of these is not considered a pure substance?
a) an atom
b) a mixture
c) an element
d) a compound

Answer;

A mixture

Explanation;A mixture is a substance that is made of two or more substances. A mixture may be homogeneous in which substances in the mixture are in the same phase, or may be heterogeneous where substances in the mixture are not in the same phase, such as suspensions and colloids. Therefore mixtures are not pure substances.A pure substance is a substance that contains atoms or molecules of the same element or compound. Examples of pure substances are elements, compounds and also atoms.