The correct answer is D. High.
Skin effect is the tendency of an alternating electric current to flow predominantly near the surface of a conductor, rather than through the bulk of the conductor. This is due to the fact that the magnetic field associated with an alternating current exerts a force on the electrons in the conductor, causing them to drift towards the surface. The higher the frequency of the current, the stronger the magnetic field and the greater the skin effect.
At very low frequencies, the skin effect is negligible and the current flows evenly throughout the conductor. At low frequencies, the skin effect is still small but can be significant in long conductors. At medium frequencies, the skin effect is more pronounced and can cause significant losses in power transmission lines. At high frequencies, the skin effect is so strong that the current can only flow on the surface of the conductor, effectively reducing the cross-sectional area of the conductor. This can lead to significant increases in resistance and power losses.
Skin effect is a major consideration in the design of high-frequency electrical equipment, such as power transmission lines and transformers. It is also important to consider skin effect when designing printed circuit boards, as it can cause uneven current distribution and lead to heating problems.