The correct answer is A) A restriction in the oil drain line to the sump. A restriction in the oil drain line would cause the oil to back up and fill the sight glass of the main turbine bearing, as the oil has nowhere else to go. This is due to the pressure differential between the bearing and the sump, which would force the oil to flow into the sight glass instead of draining properly. The other options are incorrect because: B) Excessive air trapped in the lube oil system would not cause the sight glass to be completely filled with oil. C) An increase in oil temperature would not directly lead to the sight glass being completely filled. D) Increasing the amount of oil through the gravity tank overflow line would not cause the sight glass to be completely filled, as the oil would still be able to drain properly.