Answer
$$U_{a}\gt U_{c }\gt U_{e }=U_{f}\gt U_{b }\gt U_{d }$$
Work Step by Step
We know that the electric potential of a dipole is given by
$$U_{\rm dipole}=-pE\cos\phi$$
Hence, the electric potentials of the given dipole are as follows:
$$U_{a,\rm dipole}=-pE\cos180^\circ= +pE$$
$$U_{b,\rm dipole}=-pE\cos 45^\circ= -0.707pE$$
$$U_{c,\rm dipole}=-pE\cos 135^\circ= +0.707pE$$
$$U_{d,\rm dipole}=-pE\cos 0^\circ=-pE$$
$$U_{e,\rm dipole}=-pE\cos 90^\circ=0$$
$$U_{f,\rm dipole}=-pE\cos 270^\circ=0$$
Ranking from most positive to most negative,
$$\boxed{U_{a,\rm dipole}\gt U_{c,\rm dipole}\gt U_{e,\rm dipole}=U_{f,\rm dipole}\gt U_{b,\rm dipole}\gt U_{d,\rm dipole}}$$
