Out of the given options, S-2 is not a polyatomic ion.
NO3- (Nitrate ion), C2H3O2- (Acetate ion), and CN- (Cyanide ion) are all polyatomic ions. These consist of two or more atoms covalently bonded together and carry a charge.
S-2 (Sulfide ion) is a monatomic ion. It only contains a single sulfur atom with a negative charge.
