Final answer:
The polyatomic ion in the given options is c) SO₄²-, which is known as the sulfate ion.
Step-by-step explanation:
The polyatomic ion among the options given is SO₄²-. A polyatomic ion is a charged species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered to be acting as a single unit. The charge of a polyatomic ion is not a sum of the charges on its constituent atoms, but rather the overall charge that the molecule carries.
In contrast to polyatomic ions, HCN is a linear molecule, Na⁺ is a monatomic cation, and H₂ is a diatomic molecule without any charge. Therefore, the only polyatomic ion listed is SO₄²-, which is known as the sulfate ion. Therefore, option (C) is correct.