The correct answer is C) Pressure relief bypass line around the filter. In a diesel engine lubrication system utilizing the 'full flow' principle, a pressure relief bypass line around the filter is normally provided to prevent oil starvation. This bypass line allows oil to flow directly to the engine if the filter becomes clogged, ensuring a continuous supply of lubricating oil even when the filter is restricted. The other options are incorrect because: A) A duplex strainer is used to filter the oil, but does not prevent oil starvation; B) A three-way valve is not specifically designed to prevent oil starvation; and D) A mechanical straining filter is part of the filtration system, but does not bypass the filter to prevent oil starvation.