The correct answer is C) Guarding valve (second valve away from the water drum). When performing a bottom blow on a boiler, the guarding valve (the second valve away from the water drum) is the one that should be spun open quickly. This allows the rapid release of accumulated sediment and sludge from the bottom of the boiler, which is a necessary maintenance procedure. Opening the guarding valve quickly creates a sudden pressure drop that flushes out the unwanted material. The other options are incorrect because: A) all valves should not be opened slowly during a bottom blow, as this defeats the purpose of the rapid pressure drop; B) the blow down valve (closest to the water drum) is not the one that should be opened quickly; and D) the skin valve (last before the overboard opening) is not the appropriate valve to open quickly for a bottom blow.