The correct answer is C) the suction valve is closed. If a lube oil pump fails to build up discharge pressure, the most likely cause is that the suction valve is closed. This would prevent the pump from drawing in the necessary oil, resulting in a lack of discharge pressure. The other answer choices are incorrect because: A) The bypass valve being closed would not prevent the pump from building pressure, as the oil would still be able to flow through the suction and discharge valves. B) The discharge valve being open would allow the oil to flow out of the pump, but would not prevent the pump from building pressure in the first place. D) High suction vacuum is not the cause of the pump's failure to build pressure, as the issue is likely with the suction valve itself being closed.