A compound is a substance that is made up of two or more elements that are chemically combined. A polyatomic ion is also known as a molecular ion, is a charged chemical species composed of two or more atoms. For example a nitrate ion (NO3-) is an example of a polyatomic ion in that it contains a nitrogen atom and three oxygen atoms bonded covalently to each other and act as a single charged unit. Therefore, appropriate answer would be a polyatomic ion or a molecular ion