What is the ruling on giving an answer in an exam where cheating is rampant and supervisors aid it, knowing that the questioner initially did not comply, but then gave the answer when he saw that the supervisors were not preventing it and that the exam "had no meaning"?

The ruling on cheating in exams has been previously explained, and the presence of disorganization among invigilators is not a justification for it. You erred by giving the answer to your colleague after your initial refusal, and it is obligatory to repent from that. You should have advised the supervisors if no harm would have come to you. Repent to Allah and do not return to such an act.