Why At RF, changing magnetic fields inside a conductor induce eddy currents that cancel current flow in the center and reinforce it near the surface, so the current crowds into a thin outer layer. The depth of that layer shrinks roughly as the inverse square root of frequency, which means less effective cross-sectional area is carrying the current. Less usable cross-section means higher AC resistance, and it keeps climbing as frequency climbs. This is why RF conductors are often hollow tubing, silver plated, or made of Litz wire.
Watch out The choices about temperature describe the temperature coefficient of resistance, a separate DC effect that has nothing to do with frequency, and electron mobility does not improve with frequency.
Skin effect: higher frequency, thinner skin, smaller effective wire, more resistance. Frequency, not temperature.