The correct answer is C. Skin effect.
Skin effect is a phenomenon that occurs when an alternating current (AC) flows through a conductor. The current tends to flow more on the surface of the conductor than in the center. This is because the magnetic field created by the current opposes the current itself, and this opposition is greater at the center of the conductor than at the surface. As a result, the current density is greater at the surface than at the center.
The skin effect is more pronounced for conductors with higher conductivity and for higher frequencies. It can be reduced by using a conductor with a higher resistivity or by using a conductor with a smaller diameter.
A. Permeability variation is not a factor in skin effect. Permeability is a measure of how easily a material can be magnetized. It is not affected by the flow of AC current.
B. Corona is a discharge of electricity that occurs when a high voltage is applied to a conductor. It is not related to skin effect.
D. Unsymmetrical fault is a type of electrical fault that occurs when there is an imbalance between the current and voltage in a circuit. It is not related to skin effect.