The correct answer is option [tex]NH{{+} \atop {4}} \right.[/tex].
The [tex]NH{{+} \atop {4}} \right.[/tex] is an ion made up of two atoms nitrogen and hydrogen.
The polyatomic ions are those ions, which are formed by the binding of the two or more elements, which has a charge and act as an ion. As all others are formed by a single atom, they are not polyatomic, only [tex]NH{{+} \atop {4}} \right.[/tex] is formed by two atoms, hence, it is an polyatomic ion.