What distinguishes an element from a compound?

The correct answer highlights that an element cannot be broken down into simpler substances. This is a fundamental characteristic of elements: they are pure substances made up of only one type of atom. Unlike compounds, which consist of two or more different types of atoms that are chemically bonded together, elements are the most basic units of matter. They maintain their identity and properties and cannot be separated into simpler substances through chemical means.

In contrast, the other statements do not accurately define elements. For instance, combining two or more types of atoms defines compounds, not elements. The assertion that all elements are solids at room temperature is inaccurate, as some elements like mercury are liquids and others such as bromine are gases. Lastly, the idea that an element combines substances chemically is misleading; it is compounds formed from the chemical combination of elements, rather than the elements themselves, that exhibit such behavior. Hence, the definition that emphasizes an element's inability to be broken down provides clarity on its fundamental nature.