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

2) Oxalate ion is a polyatomic ion containing carbon and oxygen with formula

3) Phosphate ion is a polyatomic ion containing phosphorous and oxygen with formula

4) Thiocyanate ion is a polyatomic ion containing sulfur, carbon and nitrogen with formula
