The correct answer is D) a low water level in the steam drum. A low water level in the steam drum can cause lower than normal steam pressure in an operating boiler. This is because the water level in the steam drum directly affects the amount of steam that can be generated. When the water level is low, there is less water available to produce steam, resulting in lower steam pressure. The other answer choices are incorrect because: A) Boiler water contamination can cause issues with boiler operation, but it does not directly lead to lower steam pressure. B) A sudden drop in superheater outlet temperature is more likely to be a symptom of lower steam pressure, rather than a cause. C) High feed water temperature can affect boiler efficiency, but it does not necessarily lead to lower steam pressure.