Which of the following is a medium commonly used for insulation in high-voltage cables?

The use of oil, particularly in the context of high-voltage cables, is significant due to its unique properties that make it suitable for insulation and cooling. In high-voltage systems, oil serves as both an electrical insulator and a coolant. It effectively prevents electrical breakdown, which can occur if the insulating material allows electric fields to become too strong. Additionally, oil can help dissipate heat generated by electrical conduction in the cables, ensuring that the system operates within safe temperature limits.

In contrast, while silicone and certain plastics may have insulating properties and can be used in various applications, they are not typically the primary medium for high-voltage cable insulation compared to oil. Air is also commonly used in insulation (such as in transmission lines where there is significant spacing), but it is generally less effective in preventing electrical breakdown at high voltages than liquid mediums like oil. Therefore, oil stands out as a common and effective choice for high-voltage cable insulation.