Why is the resistance of a conductor different for RF currents than for direct currents?

Direct current (DC) flows uniformly through the entire cross-section of a conductor. However, as the frequency of alternating current increases into the radio frequency (RF) range, the current tends to flow mostly near the surface, or "skin," of the conductor. This phenomenon is known as the "skin effect." Because the effective cross-sectional area carrying the current is reduced at high frequencies, the resistance of the conductor to RF currents is significantly higher than it is for direct currents. The other options are incorrect distractors. Conductors are fundamentally linear devices, the "Hertzberg effect" is a fabricated term in this context, and standard wire insulation acts as a dielectric rather than conducting current at these frequencies.