An anion is an atom that has

(A) gained one or more electrons.

(B) lost one or more electrons.

(C) gained one or more protons.

An anion is an atom that has A. Gained one or more electrons.

I think the correct answer from the choices listed above is option A. An anion is an atom that has gained one or more electrons making it more negative since electrons are negatively charged and adding them to a neutral atom will make the atom negative.