(a) Dipole moment is a quantity that describes two opposite charges separated by a distance. It is quantity that we can measure for a molecule in lab and thereby determine the size of partial charges on the molecule. Among the given species, more closer the groups attached to the ring → show more dipole-moments then the species that has no second group attached to the ring. Hence, increasing order of dipole moment is (iv)<(i)<(ii)<(iii) Hence, option (a) is correct.