Explanation

(a)
The dipole moment of a polar molecule depends upon its geometry. A symmetrical molecule is non-polar even though it contain polar bonds. Methane molecule $(CH_{4})$ has zero moment value of dipole moment due to its symmetrical structure.
In $CHCl_{3}$, the resultant of C – H and C – Cl dipole oppose the resultant of two C – Cl dipoles while in $CH_{2}Cl_{2}$, the resultant of C – H dipoles adds to resultant of two C – Cl. In case $CH_{3}Cl,$ the resultant of two C – H dipole adds to the resultant of two C – Cl. In case $CH_{3}Cl$ the resultant of two C – H dipoles add to the resultant of C – H and C – Cl dipoles.
Thus dipole moment of $CH_{3}Cl$ is highest among the given compounds. The molecule ($CCl_{4}$) again becomes symmetrical and dipole moment reduces to zero.