The correct answer is C) 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 steam flow and ensuring a smooth transition between the suction and discharge strokes. If the cushion valves are not properly adjusted, it can lead to an abrupt change in steam pressure, causing the pounding noise. The other answer choices are incorrect because lower than normal supply steam temperature (A) or pressure (D) would affect the overall pump performance, not just one cylinder. Loss of air charge in the pump discharge plenum chamber (B) would not specifically cause a pounding noise in one cylinder.