The correct answer is C) Suction line inlet strainer is obstructed. An abnormally deep vacuum at the oily-water separator's item "1" indicates that the suction line inlet strainer is likely obstructed. This obstruction creates a higher resistance to the flow, resulting in a deeper vacuum at the inlet. The other options are incorrect because: A) Fouled coalescer beds would create back pressure, not a deeper vacuum; B) An open process water inlet valve would not directly affect the vacuum at the inlet; and D) A high vacuum should not be maintained in the chamber whose vacuum is to be measured, as this could indicate an issue with the system.