D

Eddy currents are loops of electric current induced within conductors by a changing magnetic field. They are used in various applications such as induction furnaces (Option A) for melting metals, galvanometers (Option B) for damping the motion of the needle, and speedometers (Option C) in automobiles to measure speed. However, X-ray crystallography (Option D) does not involve eddy currents; it is a technique used to determine the atomic and molecular structure of a crystal. Thus, the application not related to eddy currents is X-ray crystallography.