Answer
c. adding or subtracting electrons to/from an atom
Work Step by Step
Adding and removing proton changes the element of atom itself, also this is not possible because protons are present in the nucleus and they can not be removed or added by chemical reactions.
Similarly neutrons are also present in the nucleus so they can not be removed also they have no charge so removing them will not create ions.
Electrons are present is outer shells and they can be added or removed to form ions.
