For which of the following molecule significant $\mathrm{\mu }\ne 0$ ?

For p-dichlorobenzene and p-dicyanobenzene, the dipole moments of individual bonds cancel each other as they are equal in magnitude and opposite in direction. Hence, the resultant dipole moment of the molecules is zero. For p-hydroquinone and p-benzenedithiol, the O−H and S−H bond dipoles in these molecules do not cancel each other as they are not in opposite directions due to existence in different conformations. Hence, the resultant dipole moment of the molecules is non zero.