The skin effect refers to the phenomenon where the flow of alternating current (AC) tends to concentrate near the surface of a conductor, rather than being uniformly distributed across the entire cross-section of the conductor. This effect becomes more pronounced as the frequency of the AC increases.

Explanation:
The skin effect is mainly influenced by the following factors:

1. Nature of Material:
The skin effect is primarily dependent on the electrical properties of the material used for the conductor. Conductors with high electrical conductivity experience a more pronounced skin effect compared to those with lower conductivity. This is because highly conductive materials offer less resistance to the flow of current, causing it to concentrate near the surface.

2. Size of Wire:
The size or diameter of the wire also affects the skin effect. As the diameter of the wire decreases, the skin effect becomes more significant. This is because a smaller cross-sectional area offers less room for the current to spread out, resulting in a concentration of current near the surface.

3. Supply Frequency:
The frequency of the AC supply is a crucial factor influencing the skin effect. As the frequency increases, the skin effect becomes more prominent. This is because at higher frequencies, the changing magnetic field induces eddy currents within the conductor, which in turn causes the current to concentrate near the surface.

4. Ambient Temperature:
Contrary to the options given, the ambient temperature does not directly affect the skin effect. The skin effect is primarily determined by electrical characteristics rather than thermal properties. However, it is important to note that temperature can indirectly impact the skin effect by altering the electrical properties of the material. For example, an increase in temperature may cause an increase in resistivity, which can affect the extent of the skin effect.

In conclusion, the skin effect primarily depends on the nature of the material, size of the wire, and supply frequency. The ambient temperature, while indirectly influencing the skin effect through its impact on electrical properties, is not a direct determinant of the skin effect.