The part of a bucket's boom tip apparatus that is electrically tested is the ___.

The correct choice is the bucket liner. In the context of a bucket's boom tip apparatus, the bucket liner serves a critical role in ensuring the integrity and performance of the equipment. It is typically made of insulated material, which is crucial for electrical safety. During testing, the bucket liner undergoes evaluations to confirm that it maintains its insulating properties effectively, ensuring that no electrical current can inadvertently pass through to the operator or the ground.

The other options do not encompass the specific components or functions that necessitate electrical testing. The outside bucket shell, while important for the overall structure and support, is not primarily focused on electrical insulation. The boom tip control relates more to the operational mechanisms than to electrical properties. The material jib, although part of the physical structure of the boom, does not play a direct role in electrical testing like the liner does. Thus, the bucket liner is the key component that is electrically tested to maintain safety standards.