The correct answer is D) the fact that no boiling occurs on heat transfer surfaces anywhere in the unit. In a two-stage flash type distilling plant, the absence of boiling on the heat transfer surfaces greatly reduces the rate of scale formation. When water boils, dissolved minerals in the water are left behind, leading to the buildup of scale on the surfaces. However, in a flash-type distillation process, the water is heated to the point of vaporization without reaching the boiling point, preventing the precipitation of minerals and the formation of scale. The other answer options are incorrect because: A) maintaining a high feed water temperature does not directly prevent scale formation, B) the pressure regulation in the first stage is not the primary factor in reducing scale, and C) operating at the unit's rated capacity does not necessarily prevent scale buildup.