Answer
Please see the work below.
Work Step by Step
Molecule (b) and (d) are symmetrical and hence their net dipole moment is zero.
Molecule (c), 1,2,4-trichlorobenzene has a dipole moment less than (a). In molecule (c) the dipole moments of 1 and 4 $C-Cl$ bonds cancel out and it possesses the dipole moment of the $C-Cl$ bond in the second position.
The increasing order of dipole moment can be written as :
$(b)=(d)\lt(c)\lt(a)$
