Which type of wiring is commonly used for underground service?

The type of wiring that is most commonly used for underground service is direct burial cable. This type of cable is specifically designed to be buried underground without the need for additional protective conduit. Direct burial cables have insulation that is robust enough to withstand moisture, soil conditions, and mechanical stress, making them suitable for underground installations where protection from external elements is critical.

This design allows for easier and more cost-effective installation, as it eliminates the need for conduit that would otherwise be required for other types of wiring. It is essential for the direct burial cables to meet certain standards and be rated for underground use, which ensures their durability and safety in such environments. The other options, while they may have specific applications, do not meet the unique demands of underground wiring as effectively as direct burial cable does.