What is the definition of an element?

An element is defined as a substance that is composed of only one type of atom. This means that all atoms within an element have the same number of protons, which is the defining characteristic that identifies the element. For example, carbon consists solely of carbon atoms, while oxygen consists only of oxygen atoms. Elements cannot be broken down into simpler substances through chemical reactions, which distinguishes them from compounds and mixtures. This fundamental property is what makes option C the correct choice in this context.

In contrast, other choices describe different types of substances—compounds consist of two or more types of atoms, substances that can be broken down chemically refer to compounds, and substances formed by chemical reactions indicate products or compounds resulting from those reactions. These definitions do not accurately describe what constitutes an element.