What is the smallest particle of a substance that retains its characteristics?

The smallest particle of a substance that retains its characteristics is an atom. An atom is the basic unit of a chemical element and is composed of protons, neutrons, and electrons. Atoms define the properties of the elements they represent; for instance, the arrangement and number of the constituent particles determine an element's physical and chemical properties.

While molecules are formed from two or more atoms bonded together, they do not represent the smallest individual unit that retains the characteristics of a substance; rather, they reflect a combination of multiple atoms. Therefore, a molecule cannot stand alone as a single entity that possesses the features of a particular substance.

In contrast, quarks are even smaller particles that combine to form protons and neutrons, but they don't have the characteristics of a substance by themselves. Elements refer to the whole class of substances that consist of one type of atom, making them larger than the individual atom itself.

Thus, the correct answer underscores the fundamental role of atoms as the smallest entities that embody the properties defining substances.