The correct answer is C) Chemically treating the incoming feed water. Chemically treating the incoming feed water is the correct answer because it can help reduce the rate of scale formation on the heating surfaces of a flash evaporator. The chemical treatment of the feed water helps to prevent the precipitation of scale-forming minerals, such as calcium carbonate and magnesium sulfate, which can deposit on the heating surfaces and reduce the efficiency of the evaporator. The other options are incorrect because they do not directly address the issue of scale formation. Decreasing or increasing the feed water heater shell absolute pressure (options A and B) would not affect the chemical composition of the feed water and, therefore, would not prevent scale formation. Chemically treating the outgoing brine (option D) may help remove some existing scale, but it would not prevent new scale from forming on the heating surfaces.