In which of the following compound/ions the $d\pi -p\pi$ is not present?

N cannot form $d\pi -p\pi$ bond with N or O.

So, the hybridization of the central atom of ${\mathrm{N}}_3^{-}$

In ${\mathrm{NO}}_2^{+}$, the nitrogen atom is bonded to one oxygen by the double bond while to the other oxygen atom by the coordinate covalent bond.

Hence, the correct option is (a, b).