Among the following, the molecule with the highest dipole moment is :

Solution :
NOTE : Dipole moment is a vector quantity.
Methane molecule being symmetrical, has zero dipole moment. Replacement of one of the $$H - $$ atoms by $$Cl$$ atom increases the dipole moment. The increase in dipole moment is rather more than what can be expected because of the fact that the bond dipole moment of $$C - H$$   bond and that of $$C - Cl$$   bond reinforce one another. Replacement of another $$H$$ atom by $$Cl$$ increases the bond angle due to lone pair - lone pair repulsion between two $$Cl - $$ atoms thereby reducing the dipole moment of the molecule. Increase in angle is again caused by the the introduction of the third $$Cl - $$ atom. When the fourth $$Cl - $$ atom is introduced, the molecule $$\left( {CC{l_4}} \right)$$  again becomes symmetrical and dipole moment reduces to zero. So, $$C{H_3}Cl$$   will have the maximum dipole moment.