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

The smallest particle of a substance that still retains its properties is an atom. Atoms are the fundamental building blocks of matter and determine the characteristics of elements and the compounds they form. Each atom retains the unique properties of its corresponding element; for example, carbon atoms will always exhibit properties characteristic of carbon, regardless of how they are arranged or bonded with other atoms.

Molecules, while they are made up of atoms and do represent a collection of atoms bonded together, can sometimes lose certain properties of the individual atoms that comprise them, especially when the molecular structure changes. Endorphins, on the other hand, are specific types of molecules that contain multiple atoms and exhibit properties distinct to their structure, but they do not represent the smallest unit of a substance. Elements consist of individual types of atoms but broader than an atom itself. Therefore, the correct choice indicates that an atom is the smallest particle that retains the unique properties of a substance.