Understand the basic definitions: A proton is a positively charged particle found in the nucleus of an atom, a neutron is a neutral particle also found in the nucleus, and a cation is a positively charged ion.
Recognize that an ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net charge.
Identify that a negatively charged ion is called an anion. This occurs when an atom gains electrons, resulting in more electrons than protons.
Compare the options given: Proton, Neutron, Cation, and Anion. Recall that protons and cations are positively charged, and neutrons have no charge.
Conclude that the only option that represents a negatively charged ion is an anion, as it has gained electrons.