Answer: (3) phosphate ion
Explanation: A polyatomic ion is a ion which contains different atoms.
1) Chromate ion is a polyatomic ion containing chromium and oxygen with formula [tex]CrO_4^{2-}[/tex]
2) Oxalate ion is a polyatomic ion containing carbon and oxygen with formula [tex]C_2O_4^{2-}[/tex]
3) Phosphate ion is a polyatomic ion containing phosphorous and oxygen with formula [tex]PO_4^{3-}[/tex]
4) Thiocyanate ion is a polyatomic ion containing sulfur, carbon and nitrogen with formula [tex]SCN^{-}[/tex]
