an element is best described as: a a substance composed of molecules only b a mixture of atoms and compounds…

Brief Explanations:

An element is a pure substance that consists of only one type of atom. It cannot be broken down into simpler substances by chemical means. Options a, b, and c are incorrect as they do not accurately describe the nature of an element. A substance composed of molecules only could be a compound, a mixture of atoms and compounds is a mixture, and a combination of two or more substances is also a mixture.

Answer:

D. A pure substance made of only one type of atom