Which one has a minimum (nearly zero) dipole moment
(A) Butene-1
(B) cis Butene-2
(C) trans Butene-2
(D) 2-Methyl-1-Propene

Hint: A bond is said to be polar when there is an electronegativity difference between the two atoms of the bond. A polar bond possesses a dipole moment. However, if the two atoms have the same electronegativity or a negligible electronegativity difference, then the bond does not possess a dipole moment. Also, if a molecule contains more than two atoms, that is, there are more than one dipole in a molecule, then the net dipole moment is the vector sum of all the dipoles in the molecule.

Complete Step by Step Answer:
(A) But-1-ene – It contains one $C-C$ double bond and one of the carbon contains ethyl groups. So, the net dipole moment is non-zero. This molecule has a dipole moment of 0.3 D.

(B) cis but-2-ene – It contains one $C-C$ double bond and each of the carbon atoms contains one methyl group. The two dipoles from methyl to carbon are in the same direction. So, the net dipole moment is non-zero. This molecule has a dipole moment of 0.33 D.

(C) trans but-2-ene - It contains one $C-C$ double bond and each of the carbon atoms contains one methyl group. The two dipoles from methyl to carbon are in the opposite direction. So, the net dipole moment is zero. This molecule has almost zero dipole moment.

(D) 2-methyl-1-propene – It contains one $C-C$ double bond and one of the carbons contains two methyl groups. This molecule has a dipole moment of 0.5 D.
Correct Option: (C) trans but-2-ene.

Note: The dipole moment is a vector quantity that has both magnitude and direction. This is why we should also keep in mind the direction of dipoles to find the dipole moment of a molecule.