The correct answer is A) there is a ground on the line with the dark lamp. In a three-phase electrical system, the ground detecting lamps are used to monitor the grounding of each phase. When the "ground test" button is pushed, a ground is intentionally introduced into the system. If one lamp goes dark while the others increase in brightness, it indicates that there is a ground on the line associated with the dark lamp. This is the normal indication of a ground fault in one of the phases. The other options are incorrect because B) this is not a normal condition, but rather indicates a problem; C) replacing the dark lamp would not resolve the underlying ground issue; and D) the bright lamps indicate proper grounding, not additional grounds on those lines.