Which of the following has/have zero dipole moment?

In trans-1,2-dichloroethene and 1,4-dichlorobenzene the two C-Cl dipoles are equal and opposite, giving zero. In 1,2-dichlorobenzene and 1,1-dichloroethene the C-Cl dipoles are at an angle ($60^{\circ }$ and $120^{\circ }$), so the resultant is non-zero.