The correct answer is D) observing the overflow sight glass. In a gravity-type lube oil system, the main lube oil pump draws oil from the gravity tanks. After starting the pump, the proper way to verify that the gravity tanks are full is to observe the overflow sight glass. This allows you to confirm that the tanks are at the proper level and that the oil is circulating through the system as intended. The other options are incorrect because sounding the tanks (A) or the sump (C) would not directly indicate whether the gravity tanks are full, and observing the flow from the bearings (B) would not provide a clear indication of the gravity tank level.