Which of the following is not an example of a molecule? a: mn b: koh c: h2s d: o3

Mn
This is an atom of Manganese

The second one is a compound of potassium oxygen and hydrogen, the third one is a compound of 2 hydrogens and a sulfer and the fourth one, while only containing one element, is still a molecule of 3 oxygen atoms - leaving Mn as your answer.

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).