Why one of the three structures for c2h2cl2 is nonpolar?

Below are the three possible isomers of chemical formula C₂H₂Cl₂. Among these three isomers the trans-1,2-dichloroethene is non-polar while, remaining two are polar.

Reason:

Reason for this is dipole moment. In case of cis-1,2-dichloroethene and 1,1-dichloroethene, the vectors are reinforcing each other perpendicularly to double bond, hence, giving a net dipole moment and making these two isomers polar in nature.

While, in case of trans isomer the two vectors are cancelling each other as the two chlorine atoms are in opposite directions. Hence, the net dipole moment is zero in this case. So, it is non-polar in nature.