What are the fundamental substances that cannot be broken down by chemical methods?

The correct choice is based on the definition of elements in chemistry and biology. Elements are pure substances that consist of only one type of atom, and they cannot be broken down into simpler substances by chemical means. Each element is represented on the periodic table and has unique properties that define it.

In contrast, molecules and compounds consist of two or more atoms bonded together. Molecules may be made up of the same type of atom (like O2, which is oxygen gas) or different types (like H2O, which is water). While these can be broken down into their constituent elements through chemical reactions, elements themselves remain indivisible in this manner.

Atoms, the basic units of elements, are also not subject to chemical breakdown but rather are the fundamental building blocks that make up elements. Therefore, while atoms and elements relate closely, it's the element as a whole that is defined as a basic substance that cannot undergo chemical decomposition. This distinction highlights why elements are the fundamental substances referred to in the question.