The correct answer is B) brine density is too low. The efficiency and output capacity of a low pressure distilling plant are directly affected by the brine density. If the brine density is too low, it can reduce the efficiency and output capacity of the distilling plant. This is because the brine density plays a crucial role in the heat transfer and evaporation process within the distilling plant. When the brine density is too low, the heat transfer and evaporation are less effective, leading to a decrease in the overall efficiency and output capacity of the system. The other answer choices are incorrect because they do not directly impact the efficiency and output capacity of the distilling plant in the same way as a low brine density. Maintaining a steady pressure (A), avoiding rapid pressure fluctuations (D), or ensuring the brine density exceeds a specific threshold (C) are not the primary factors that determine the efficiency and output capacity of the low pressure distilling plant.