Compare the four options against what is physically happening on the flat surface of an oil film.
Dispersion is the splitting of white light into separate colours by a prism, caused by different colours bending by different amounts. There is no prism shaped bending here, so this does not fit.
Diffraction is the bending of light around edges, corners, or through narrow slits. An oil film has no edge or slit involved, so this does not fit either.
Polarization restricts the vibration direction of light waves but does not by itself create a pattern of colours.
Interference happens when two reflected waves, one from the top of the film and one from the bottom, combine and reinforce some wavelengths while cancelling others, depending on the film thickness at that point.
This matches the changing, rainbow like colours seen on oil films and soap bubbles, so option B, interference, is correct.