Answer:
The correct answer is D. An ion is an atom that has lost an electron.
Step-by-step explanation:
An ion is an electrically charged particle consisting of an atom or molecule that is not electrically neutral. Conceptually this can be understood as that, from a neutral state of an atom or molecule, electrons have been gained or lost; This phenomenon is known as ionization.
When an atom loses or gains electrons, the species formed is an ion and carries a net electric charge. Since the electron has a negative charge, when one or more electrons are added to an electrically neutral atom, a negatively charged ion is formed. Losing electrons produces a positively charged ion. The number of protons does not change when an atom becomes an ion.