Answer: Option (a) is the correct answer.
Explanation:
Poly means many and polyatomic ions means an ion(s) with two or more different atoms.
Out of the given options, phosphate ion is the polyatomic ion which has an ionic charge of 3-.
The formula depicting a phosphate ion is .
Whereas Nitrate, Sulfate, Hydroxide , Bicarbonate does not carry or have a 3- ionic charge.