A substance that cannot be broken down by chemical means is known as what?

The correct answer is "an Element" because an element is defined as a pure substance that cannot be decomposed into simpler substances through chemical reactions. Elements consist of only one type of atom, and they are the fundamental building blocks of matter. Each element is represented in the periodic table and has unique properties.

An atom, while being the smallest unit of an element, can exist in combination with other atoms to form molecules and isn't referred to as a substance in the same context as an element. A compound is a chemical substance formed from two or more elements that are chemically bonded together. Solutions, on the other hand, are homogeneous mixtures of two or more substances and can be separated by physical means, not chemical. Thus, understanding the definition of an element highlights its uniqueness as something that cannot be broken down chemically, making it the correct answer to the question.