The Dipole Moment ($\mu$) is a vector quantity. A molecule has zero net dipole moment if its individual bond dipoles cancel out due to symmetry.

1. $C{O}_2$: Linear structure ($O=C=O$). The two $C=O$ bond dipoles are equal in magnitude but act in exactly opposite directions ($180^{\circ }$). Resultant $\mu =0$.
2. $H_{2}O$: Bent shape. Bond dipoles don't cancel. $\mu \ne 0$.
3. $N{H}_3$: Trigonal pyramidal shape. Resultant $\mu \ne 0$.
4. $HCl$: Polar diatomic molecule. $\mu \ne 0$.