There are two forms of 1,2-dichloro-ethene.
One is called cis-1, 2-dichloroethene, and the other is called trans-1,2-di-chloroethene.
I. The boiling point is higher for cis-1, 2-dichloroethene because the dipole moment for cis-1, 2-dichloroethene is higher than for trans-1,2-di-chloroethene.
II.