Which of the following best describes a polyatomic ion?

A polyatomic ion is indeed best described as a group of atoms that possess a positive or negative charge. This definition captures the essence of polyatomic ions, which consist of multiple atoms that are covalently bonded together and collectively gain or lose electrons, resulting in a charge. The presence of this charge allows polyatomic ions to participate in ionic bonding with other charged species.

In contrast, an atom that has lost or gained electrons refers specifically to a monatomic ion, which does not apply to polyatomic ions since they are made up of multiple atoms. The description of a single atom with no charge pertains to neutral atoms and does not reflect the charged nature of polyatomic ions. Additionally, a molecule formed from two metals generally describes metallic bonding and does not represent the structure or composition of a polyatomic ion, which typically contains nonmetals or a combination of nonmetals and metals.