Which of the following has a net dipole moment ?

The geometry of SO₂ is bent because the lone pair on the central sulphur atom pushes the two bonds down for a bond angle of 120^o due to the electrons repulsion. This is what makes of SO₂ a polar molecule because the dipole moments of both bonds of not cancel out, the molecule will have a net dipole moment.

Hence, the correct option is option (A) ${\mathrm{NO}}_3^{-}$.