Which of the following is not permitted to be used as a grounding electrode?

The correct choice is based on the regulations outlined in the National Electrical Code (NEC) regarding grounding and bonding. A metal underground gas piping system is not considered a suitable grounding electrode because it is not electrically conductive for grounding purposes and is subject to corrosion. The NEC specifies that grounding electrodes must be effective in providing a low-resistance path to ground, and the integrity of gas piping systems can be compromised over time due to their function and materials, leading to potential safety hazards.

Conversely, driven ground rods, concrete footings, and metal underground water pipes are recognized as appropriate grounding electrodes. A driven ground rod provides a direct connection to the earth, while a concrete footing can serve as part of the grounding system as long as it is in contact with the soil. A metal underground water pipe, if it meets specific length and continuity criteria, can also be effectively utilized as a grounding electrode, thus ensuring safety and grounding effectiveness.