What tool would be most useful for installing underground wiring?

The trencher is the most useful tool for installing underground wiring because it is specifically designed to create narrow, deep trenches efficiently. When installing wiring underground, it is essential to maintain a precise depth and width for the trench to protect the wiring and ensure it is below the frost line and other interfering elements. A trencher is capable of achieving this more accurately and with less physical effort than other tools.

Other tools serve different purposes and may not meet the specific requirements for underground wiring installation. For instance, a shovel is primarily used for digging and moving material but lacks the precision and consistency a trencher provides. An auger is designed more for drilling into the ground and may not create the necessary continuous trench needed for wiring. A trowel is typically utilized for smaller-scale gardening tasks and isn't suited for trenching applications at all. Thus, the trencher stands out as the optimal choice for safely and effectively installing underground wiring.