What solution is required to maintain a constant temperature in the brine tank?

To maintain a constant temperature in the brine tank, the appropriate solution involves managing the saturation and crystallization aspects of the brine. The use of a recrystallization solution helps ensure that the salt remains in an optimal state for effective ion exchange and minimizes issues related to temperature fluctuations. This environment prevents the formation of unwanted crystal structures that could impair the efficiency of the system.

In contrast, options like Demand Flow Solution, Thermal Insulation, and Cooling System do not specifically address the temperature stability in relation to the brine's specific requirements. While thermal insulation may help reduce temperature fluctuations, it does not actively manage the brine saturation level effectively, which is crucial for performance. A cooling system might cool the brine tank but would not be necessary if the brine is appropriately maintained through recrystallization. Thus, the use of a recrystallization solution presents the most effective method for ensuring that the temperature remains constant and optimal for function.