The correct answer is A) An alarm will sound. In a gravity-type lube oil system, the main lube oil pump is responsible for maintaining the necessary oil pressure throughout the system. If the pump discharge pressure is lost, the first event that will occur is the activation of an alarm to notify the crew of the pressure loss. This is a critical safety feature, as the loss of lube oil pressure can lead to serious damage to the bearings and gears if not addressed promptly. The other options are incorrect because: B) The astern throttle will not immediately open, as this is not a typical response to a loss of lube oil pressure. C) Lube oil will not be provided to the bearings and gears via the gravity tank overflow line, as the system is designed to rely on the main lube oil pump for proper lubrication. D) All bearing oil pressure will not be lost immediately, as the gravity tank can provide a temporary supply of oil until the issue is resolved.