Which term best describes a substance that cannot be broken down into simpler substances?

The term that best describes a substance that cannot be broken down into simpler substances is "Element." An element is a pure substance that consists of only one type of atom and cannot be separated into simpler components through chemical processes. Each element is defined by its number of protons in the nucleus, which determines its identity and properties.

In contrast, a molecule is formed when two or more atoms bond together, which means it can consist of either elements or compounds. A mixture consists of two or more substances that are combined but not chemically bonded, allowing for their separation by physical means. A compound is made up of two or more different elements that are chemically bonded together, and while it can be broken down into its constituent elements through chemical reactions, it is not an element in itself. Thus, the unique characteristic of an element as a fundamental building block of matter distinguishes it as the correct term in this context.