The polyatomic ion out of the given options is hydroxide ion that consists of two atoms.
Polyatomic ions are the one that are composed of two or more than two atoms/elements.
The formula for Iron (II) ion is Fe2+.
The formula for Chloride ion is Cl-.
The formula for magnesium ion is Mg2+.
The formula for hydroxide ion is OH-.
In hydroxide ion, there are two atoms i.e., one hydrogen and one oxygen atom. Therefore, it is considered as a polyatomic ion.
Thus, option b is the correct choice.
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