The correct answer is A) The oily-water separator bilge suction strainer is clogged. The reasoning is that a clogged oily-water separator bilge suction strainer would cause an unusually deep vacuum in the separator vessel, as the pump would have to work harder to pull the water through the restricted strainer. This would result in the observed unusually deep vacuum reading, even with the separator service pump running. The high vacuum indicates a restriction in the system, and a clogged strainer is the most likely cause. The other options are incorrect because a leaking relief valve would not cause a vacuum, a worn pump would likely result in lower vacuum, and a high bilge tank level would not directly impact the vacuum in the separator vessel.