The correct answer is D. All of the above.
Skin effect is the tendency of an alternating electric current to flow predominantly near the surface of a conductor, rather than through the entire cross-section of the conductor. This is due to the interaction between the electric field and the magnetic field produced by the current. The electric field causes the electrons to drift towards the surface of the conductor, while the magnetic field causes them to circulate around the conductor. This circulation of electrons creates a secondary electric field that opposes the original electric field, which further concentrates the current near the surface.
The skin depth is the depth at which the current density falls to 1/e (about 0.368) of its value at the surface. The skin depth is inversely proportional to the square root of the frequency of the current and the resistivity of the conductor material, and directly proportional to the square root of the conductor’s radius.
Therefore, the skin effect depends on the frequency of the current, the size of the conductor, and the resistivity of the conductor material.