How far should ground rods be driven into the ground for proper installation?

For proper installation of ground rods, the standard requirement is that a certain length of the rod should remain exposed above the ground. Having 3 inches of the top of the ground rod exposed allows for effective grounding while ensuring accessibility for future maintenance or testing. This amount also facilitates the attachment of grounding conductors to the rod without compromising conductivity or the rod's effectiveness in providing a reliable ground connection.

The other options indicate varying degrees of exposure that are either insufficient or excessive for practical use. An exposed length that is too short may not provide enough area for proper grounding connections, while too much exposure could be unnecessary and may lead to mechanical vulnerabilities. Therefore, 3 inches is the optimal requirement for practical and effective grounding system functionality.