Answer: Atom
Explanation:
The atom is the smallest unit that can be found of an element so the atom is the correct answer. Atoms are the smallest because the atom retains the properties of the element. Anything smaller than an atom will not retain those qualities.
Atoms have electrons, protons, and neutrons with the last two existing in the nucleus of the atom but these cannot be said to be the smallest units because they do not have gold properties individually.