Which of the following cannot be used as a grounding electrode?

The correct option is based on the principle that grounding electrodes are designed to provide a low-resistance path to ground for electrical systems, ensuring safety and proper operation.

A metal water pipe is typically an acceptable grounding electrode because it is conductive and often buried in the ground, which can establish a strong electrical connection with the earth. A concrete-encased electrode, often consisting of reinforcing bars in concrete, also serves effectively as a grounding electrode due to the conductive materials involved.

A ground rod, usually made of copper or coated with copper, is a common and widely used device for grounding, as it is specifically designed to be driven into the ground to facilitate grounding connections.

On the other hand, a tree does not meet the criteria to be considered a proper grounding electrode. Trees may not possess uniform conductivity and can vary significantly in their moisture content, species, and structure. This inconsistency makes them unreliable as a grounding method. Additionally, using a tree as a grounding electrode could pose safety risks, as it does not provide a direct or predictable path for electrical faults to dissipate into the ground. Thus, it is not suitable for grounding purposes per electrical codes and safety standards.