Why the dipole moment of CD3-F is more than CH3-F ???

Dipole-moment signifies the electrical behaviour of all electron pairs in the molecule, not just the bonding pair. Therefore the dipole moment cannot tell us quantitatively the difference between the electro negativities of two bonded atoms. given in the question. Both H and D have one proton and one electron, the difference is D has one neutron. This makes its atomic radius larger than H and consequently less electronegative than H , Its electronegativity decreases because the distance of bonding pair of electrons in D from nucleus are larger than H consequently less attraction of nucleus.