The correct answer is C) completely drain the unit. When securing a jacket water heated submerged tube evaporator for an extended period, it is important to completely drain the unit to prevent corrosion and damage. Leaving water in the unit can lead to the formation of scale and rust, which can degrade the equipment over time. Draining the unit completely ensures that no residual water remains that could cause issues during the extended downtime. The other options are incorrect because: A) filling with salt water would accelerate corrosion, B) a descaling compound is not necessary for storage, and D) sealing the unit without draining could still allow water to remain and cause problems.