The dipole moment of a polar molecule depends on its geometry and shape.
A symmetrical molecule is non- polar even though it contains polar bonds.
$CH_4$ being symmetrical molecule has zero resultant dipole moment.
We know that the bond dipole moment of $C-H$ bond and that of $C-Cl$ bond reinforce one- another.
In $CHCl_3$ the resultant of $C-H$ and $C-Cl$ dipoles while in $CH_2Cl_2$ the resultant $C-H$ dipoles add to the resultant of two $C-Cl$ dipoles.
In case of $CH_3Cl$ the resultant of two C-H dipole s add to the resultant of $C-H$ and $C-Cl$ dipoles.
Thus dipole moment of $CH_3CL$ is highest among the given compounds.
The module $(CCl_4)$ again becomes symmetrical and dipole moment reduces to zero.
Hence a is the correct answer.