Introduction
The skin depth phenomenon refers to the behavior of electromagnetic waves as they penetrate a material. It is a measure of how deeply the waves can penetrate into the material before their intensity decreases significantly. The skin depth is influenced by the electrical conductivity of the material and the frequency of the electromagnetic waves. In this context, the correct answer is option 'C' - Conductors.

Explanation
The skin depth phenomenon is primarily observed in conductors. Conductors are materials that have high electrical conductivity, allowing them to easily conduct electric current. When an electromagnetic wave interacts with a conductor, it induces electric currents within the material. These currents, known as eddy currents, circulate near the surface of the conductor.

Factors Affecting Skin Depth
Several factors influence the skin depth in conductors:

1. Electrical Conductivity: Higher conductivity leads to a smaller skin depth. Materials with low conductivity, such as insulators, have larger skin depths.

2. Frequency: Higher frequencies result in smaller skin depths. This is because at higher frequencies, the electromagnetic waves penetrate the material less effectively.

3. Permeability: The magnetic permeability of the material also affects the skin depth. Higher permeability results in an increased skin depth.

Applications
The skin depth phenomenon has important applications in various fields:

1. Power Transmission: In power transmission lines, high-frequency alternating currents are used. By understanding the skin depth, engineers can design conductors with appropriate dimensions to minimize power loss due to skin effect.

2. Shielding: Conductive materials with low skin depth are used for electromagnetic shielding. The skin depth prevents electromagnetic waves from penetrating the shield and reaching the protected area.

3. Electromagnetic Compatibility: The skin depth is crucial in designing electronic circuits and devices to ensure proper functioning and minimize interference from external electromagnetic fields.

4. Induction Heating: Induction heating relies on the skin effect to heat conductive materials. By inducing alternating currents near the surface of the material, heat is generated.

Conclusion
The skin depth phenomenon is primarily observed in conductors. It is influenced by factors such as electrical conductivity, frequency, and permeability. Understanding the skin depth is essential in various applications, including power transmission, electromagnetic shielding, and induction heating.