The correct answer is D) change the oil flow over to the clean side. The reasoning is that on a duplex fuel oil strainer system, when one side becomes clogged, the proper first action is to switch the fuel oil flow over to the clean side of the strainer. This allows the engine to continue operating without interruption while the clogged strainer can be cleaned at a more convenient time. Stopping the fuel oil service pump or opening the strainer bypass valve would interrupt fuel flow to the engine, which is not the desired initial response. Quickly cleaning the dirty strainer is also not the best first step, as the engine should be kept running if possible.