Dipole moment is shown by
cis-1, 2-dichloroethene
The explanation for the correct option:
(C) A dipole moment is a measurement of the separation of two oppositely charged charges. The polarity of the cis compound is more than the trans compound. Because in trans, the same bonds are exactly opposite each other thus making dipole moments equal to zero but cis gives the resultant dipole moment.
cis-1, 2-dichloroethene is a cis isomer and has a dipole moment.
The explanation for the incorrect options:
(A) trans-2 and 3-dichloro-2-butene have the same bond and group attached exactly opposite to each other, making dipole moment zero.
(B) 1, 4-dichlorobenzene have the same bond and group attached exactly opposite to each other, making dipole moment zero.
(D) trans-1, 2-dichloroethene have the same bond and group attached exactly opposite to each other, making dipole moment zero.
Hence, option (C) is the correct option. Dipole moment is shown by cis-1, 2-dichloroethene.