The correct answer is D) Sufficient oil flow is being supplied to the gravity tank. When oil is flowing through the lube oil gravity tank overflow sight glass, it indicates that the oil flow to the gravity tank is sufficient. The gravity tank is designed to maintain a constant oil level and any excess oil will flow out through the overflow, which can be observed through the sight glass. This ensures that the bearings and other critical components are receiving the necessary lubrication. The other options are incorrect because: A) Excessive oil would cause the level in the tank to rise, not flow through the overflow; B) Insufficient oil flow would result in the level dropping, not overflowing; and C) Turbine bearing failure would not directly affect the oil flow to the gravity tank.