The correct answer is D) spade blanks. Spade blanks are solid metal plates that are inserted between the flanges of a pipeline to physically isolate one section of the system from another. This is the appropriate method for isolating fire hydrant valves on a crude oil tanker from the crude oil washing system, as it provides a positive physical barrier to prevent the flow of crude oil. The other options are not suitable for this purpose. Automatic closing valves would not provide a full physical isolation, non-return valves would only allow one-way flow, and face blanks are typically used for temporary isolation rather than permanent separation of systems.