Final answer:
The ammonium ion (NH4+) is classified as a polyatomic cation because it consists of multiple atoms covalently bonded together with an overall positive charge. So, the correct option is C.
Step-by-step explanation:
The correct classification for the ammonium ion (NH4+) is polyatomic cation. A polyatomic ion is defined as a group of atoms that are covalently bonded together and which carry an overall electrical charge. As the ammonium ion is formed from one nitrogen atom and four hydrogen atoms bonded together with a positive charge, it fulfills the criteria for being a polyatomic ion. It is specifically a cation because it has a positive charge.