The correct answer is A) ensure distillate quality and distilling plant efficiency. The quantity and density of brine discharged from the last effect of a flash type evaporator should be kept at a constant value to ensure the quality of the distillate produced and maintain the overall efficiency of the distilling plant. This is because the brine concentration and density directly impact the heat transfer and evaporation process in the evaporator, which in turn affects the purity and yield of the distillate. The other options are incorrect because they do not directly address the primary purpose of maintaining constant brine discharge, which is to optimize the distillation process and product quality. Option B is incorrect as it focuses on the distillate outlet temperature, while option C addresses scale prevention in the heat exchanger, and option D is incorrect as it aims to prevent any salinity in the distillate, which is not the primary goal of the brine discharge control.