Which of the following best defines an element?

An element is best defined as a pure substance that cannot be broken down into simpler substances by chemical means. This definition highlights that elements are the fundamental building blocks of matter; they consist of only one type of atom and retain their unique properties. For instance, elements such as hydrogen, oxygen, and carbon cannot be decomposed into any substances that would retain the characteristics of the original element.

The other options describe concepts that are distinct from what constitutes an element. A substance that can be chemically broken down refers to compounds, which consist of two or more elements that are chemically bonded together. A mixture of different substances indicates a physical combination of various materials, where the individual components maintain their properties and can be separated by physical means. Finally, a material composed of multiple elements generally refers to compounds or alloys, which do not fall under the definition of an element since they combine two or more elements. Therefore, option C accurately captures the essence of what an element is.