Which of the following is a pure substance that cannot be broken down by physical means?

An element is defined as a fundamental substance that cannot be broken down into simpler substances by physical or chemical means. Elements are the basic building blocks of matter, represented on the periodic table, and each element consists of only one type of atom. This characteristic distinguishes elements from compounds, which are made up of two or more different types of atoms chemically bonded together, and mixtures or solutions, which consist of two or more substances that retain their individual properties and can be physically separated.

In the context of the other options, a compound is formed when two or more elements chemically combine and can be broken down into simpler substances through chemical reactions. Mixtures are combinations of two or more substances that do not chemically bond, and their components can be separated by physical methods. A solution is a specific type of mixture where one substance (the solute) is dissolved in another (the solvent), and again, the components can be separated by physical means. Therefore, the defining characteristic of an element as a pure substance that cannot undergo physical breakdown underlines its selection as the correct answer.