The correct answer is A) completely drain the unit. When securing a flash-type evaporator for an extended period, it is important to completely drain the unit of all water. This prevents the formation of scale and corrosion inside the evaporator, which can damage the unit over time. Leaving water in the evaporator can lead to mineral buildup and potential damage. The other options are incorrect because: B) filling with descaling compound is unnecessary if the unit is properly drained, C) filling with salt water would promote corrosion, and D) sealing the unit without draining it first would still allow scale and corrosion to occur.