What is defined as a specific substance that cannot be broken down into simpler substances by chemical reactions?

The correct choice is element, as it refers to a fundamental substance that consists of only one type of atom and cannot be chemically decomposed into simpler substances. Elements are the basic building blocks of all matter and are organized in the periodic table, where each element is defined by its atomic number, which is determined by the number of protons in its nucleus.

Compounds are formed when two or more different elements are chemically bonded together, thus making them a combination of elements rather than a singular, indivisible substance. Molecules are collections of atoms that may consist of the same or different elements, but they can be broken down into simpler substances involving their atomic components. Ions are charged particles that result from the loss or gain of electrons by atoms, which also indicates that they are not fundamental substances but rather derived from elements. Hence, element is the only term that accurately describes a substance that cannot be broken down into anything simpler through chemical means.