Dipole moment is a measure of the polarity of a molecule. It indicates the separation of positive and negative charges within a molecule. A molecule with a dipole moment has a positive end and a negative end, similar to the positive and negative poles of a magnet.

Explanation of option B - cis-1,2-dichloroethane:
- In cis-1,2-dichloroethane, the two chlorine atoms are present on the same side of the molecule.
- Chlorine is more electronegative than carbon, so it pulls the electron density towards itself, creating a partial negative charge on the chlorine atom and a partial positive charge on the carbon atom.
- This separation of charges results in a dipole moment, with the chlorine atom being the negative end and the carbon atom being the positive end.

Explanation of option D - trans-1,2-dichloro-2-pentene:
- In trans-1,2-dichloro-2-pentene, the two chlorine atoms are present on opposite sides of the molecule.
- Chlorine is more electronegative than carbon, so it pulls the electron density towards itself, creating a partial negative charge on the chlorine atom and a partial positive charge on the adjacent carbon atom.
- This separation of charges results in a dipole moment, with the chlorine atom being the negative end and the carbon atom being the positive end.

Key Points:
- Dipole moment is a measure of the polarity of a molecule.
- A molecule with a dipole moment has a positive end and a negative end.
- In cis-1,2-dichloroethane, the two chlorine atoms are present on the same side of the molecule, creating a dipole moment.
- In trans-1,2-dichloro-2-pentene, the two chlorine atoms are present on opposite sides of the molecule, creating a dipole moment.