Molecules are… a) mixtures b) a pure substance c) not a pure substance

Molecules are pure substances.  This is because a molecule has a set ratio of elements in order to make that compound have the same properties.

For example:

Water is H₂O.  Always.  There is no alternate water formula.

Sand however, is a mixture, who knows exactly what is in sand, and how much of it is in there.

the answer is b. dilution