The correct answer is d. atom.
An atom is the basic unit of a chemical element. It is the smallest particle of an element that retains all the chemical properties of that element. Atoms consist of a nucleus made of protons and neutrons, surrounded by a cloud of electrons. While protons, neutrons, and electrons are the subatomic particles that make up an atom, they do not exhibit the full characteristics of the element on their own. Only when these particles come together to form an atom do they manifest the unique properties associated with that element, such as reactivity, atomic mass, and behavior in chemical reactions.