Which of the following options describes a digger that primarily relies on electrical power?

The option that identifies a digger primarily reliant on electrical power is indeed electric. Electric diggers utilize electric motors to operate, which allows them to perform digging tasks with the power supplied from an electrical source. This type of equipment is known for being quieter and often more efficient than their counterparts that rely on fuel combustion or hydraulic systems.

In contrast, pneumatic diggers operate using air pressure and are more commonly associated with tools like jackhammers. Mechanical diggers might refer to any machinery that utilises a physical mechanism but does not specify the power source and is often powered by combustion engines. Hydraulic systems, while capable of offering considerable power for digging through the use of hydraulic fluid, do not depend on electrical power as their primary energy source. Thus, electric is the designation that clearly points to its reliance on electricity.