The correct answer is B) improper adjustment of steam cushion valves. A pounding noise in one cylinder of a steam reciprocating general service pump is typically caused by improper adjustment of the steam cushion valves. These valves are responsible for controlling the amount of steam cushion at the end of the piston stroke, and if they are not properly adjusted, it can result in excessive pounding or knocking in that cylinder. The other answer choices are incorrect because lower than normal supply steam pressure (A) or temperature (D) would typically affect the performance of the entire pump, not just one cylinder. A loss of air charge in the pump discharge plenum chamber (C) would not specifically cause a pounding noise in one cylinder.