Concept:Efflorescence is the loss of water of crystallization by a hydrated salt when exposed to air.Explanation:Some crystalline salts contain water of crystallization in their structure.When such salts are left exposed to the atmosphere, they may lose this water naturally.This process occurs because the salt gives up water molecules to the surrounding air.The correct term for this loss of water on exposure to the atmosphere is efflorescence.Deliquescence, effervescence, and fluorescence describe different phenomena and do not involve loss of water of crystallization.Answer:A. efflorescence