Explanation:
The skin effect is a phenomenon that occurs in conductive materials, such as the resistance wire used in resistance standards, when they carry high-frequency alternating current (a.c.). It causes the current to concentrate near the surface of the conductor, reducing its effective cross-sectional area and increasing its resistance. This effect becomes more significant as the frequency of the current increases.

In order to reduce the skin effect in resistance standards when they are used on high-frequency a.c., the diameter of the resistance wire should be small. This is because the skin effect is more pronounced in conductors with larger diameters. By using a smaller diameter wire, the surface area of the conductor is reduced, minimizing the concentration of current near the surface and reducing the skin effect.

Reasons for the Correct Answer:

Diameter of the Resistance Wire:
The diameter of the resistance wire should be small in order to reduce the skin effect. When the diameter is large, the current tends to concentrate near the surface, resulting in an increase in effective resistance. By using a smaller diameter wire, the current is distributed more evenly across the cross-section of the wire, reducing the skin effect.

Adjacent Turns Carrying Current in Opposite Direction:
This option is incorrect because adjacent turns carrying current in the opposite direction does not directly address the skin effect. While it is true that current flowing in opposite directions in adjacent turns creates a cancelling effect for magnetic fields, it does not significantly reduce the skin effect.

Providing Four Terminals:
This option is also incorrect because providing four terminals does not directly address the skin effect. Four-terminal sensing is a technique used to measure the resistance of a specimen accurately by separating the current and voltage sensing points. It does not have a direct impact on reducing the skin effect.

Conclusion:
In order to reduce the skin effect in resistance standards when they are used on high-frequency a.c., the diameter of the resistance wire should be small. This ensures that the current is distributed more evenly across the wire's cross-section, minimizing the concentration near the surface and reducing the skin effect.