The dipole moment of

is 1.5 D.

The dipole moment of  is

In the given structure, 

Dipole moment of 2 Cl are canceled because their net dipole moment get canceled. Rest of two dipole moment is calculated as:

${\mu }^{\text{2}}={\mu }_1^2+{\mu }_2^2+2{\mu }_1{\mu }_2\cos \theta$

= ${\left(1.5\right)}^2+{\left(1.5\right)}^2+2\left(1.5\right)+\left(1.5\right)\cos 120$

= ${\left(1.5\right)}^2+{\left(1.5\right)}^2-2\left(1.5\right)\left(1.5\right)\left(\frac{-1}{2}\right)={\left(1.5\right)}^2$

             $\mu =1.5$