A polyatomic ion is an ion composed of two or more atoms that are covalently bonded together and carries a net charge. Among the options you provided:
- Mg²⁺ is a monatomic ion (magnesium ion).
- O²⁻ is also a monatomic ion (oxide ion).
- NH₄⁺ is a polyatomic ion (ammonium ion).
- Na⁺ is a monatomic ion (sodium ion).
Thus, the polyatomic ion from your list is NH₄⁺.