What effect causes most of an RF current to flow along the surface of a conductor?

The tendency of high-frequency alternating current, such as Radio Frequency (RF), to flow primarily along the outer edge of a conductor is called the "skin effect." The opposing magnetic fields generated by the AC current cancel out the current flow in the center of the wire, forcing electrons to the surface or "skin" of the conductor. Other options are entirely different scientific phenomena. For instance, the piezoelectric effect refers to generating a voltage from mechanical stress (like in crystal oscillators), and resonance effect relates to tuned LC circuits. The "layer effect" is simply a made-up distractor term designed to sound plausible.