Correct option is C
In interferometry, the phenomenon primarily used to measure very small linear displacements with high precision is interference. Interferometers work by splitting a beam of light into two paths, reflecting them back, and then recombining them to produce an interference pattern. Any minute displacement in one path changes the phase difference, causing observable shifts in the fringes, which can be measured with extreme accuracy. Diffraction, polarization, and refraction are related optical phenomena but are not the basis for precise displacement measurements in interferometry.
