Answer
Please see the work below.
Work Step by Step
Molecule (b) has a greater dipole moment than (a).
Molecule (a) and (b) are cis-trans isomers. Molecule (a) is the trans isomer and molecule (b) is the cis.
In molecule (a), the dipole moment of $C-Br$ bonds are in opposite directions and hence canceled out. Thus net dipole moment of the molecule (a) is zero.
In molecule (b), the dipole moment of $C-Br$ bonds are in the same direction and hence adds up. Therefore molecule (b) has a net dipole moment.
