Answer
Choice E. The indices of refraction of the materials on either side of the film need to be specified.
Work Step by Step
If the film’s index of refraction is lower/higher than that of the material on both sides of itself, a $180^{\circ}$ phase shift will occur at one interface but not at the other.
However, if the film has a lower-index material on one side and a higher-index material on the other, then phase shifts occur at neither interface, or at both interfaces.
None of the statements A-D will always hold; It depends on how the film's index of refraction compared to those of its neighbors.
