The correct answer is D) On the deck outside the pump room. The reasoning for this is that combustible gases will accumulate in the lower levels of an enclosed space like a pump room, due to the higher density of these gases compared to air. Therefore, the combustible gas indicator should be placed at the lowest level to accurately detect the presence and concentration of any combustible gases present. Placing it on the deck outside the pump room, rather than inside, helps ensure the safety of the personnel conducting the testing, in case high concentrations of combustible gases are detected. The other options, A) in the upper level, B) in the middle level, and C) in the lower level of the pump room, are incorrect because they do not account for the tendency of combustible gases to accumulate in the lowest part of the enclosed space.