What signifies that a metal part must be connected to ground?

The correct choice is based on the principle that any metal part with a high potential of electrical charge poses an increased risk of electric shock or fire. Connecting such metal parts to ground serves as a safety measure to redirect excess electrical energy safely into the ground, preventing hazardous situations. By grounding metal components that could carry significant charge or become energized, the risk of electric shock to personnel working near or on these equipment is minimized.

Grounding also helps in systems protection; it can prevent damage caused by electrical surges or short circuits by allowing excess voltage to safely dissipate. This safety practice is essential in various environments, particularly in electrical installations, where metal parts may inadvertently become energized due to faults, setting ground connections as a critical safety requirement.

Other options, while they might have relevance in certain contexts, do not directly address the central safety concern that grounding is intended to resolve. For example, a part being located above ground level or being part of service equipment does not inherently necessitate a ground connection unless the potential for electrical charge exists. Similarly, having an insulated coating could still mean that if there is any fault in the insulation system, there is a risk that the part could become charged.