Which one of the following impression materials must be stored in a damp state once it has been used to take an impression to prevent distortion?

The correct answer is alginate. Once alginate impressions are taken, they must be stored in a damp state to prevent distortion. This is because alginate is a hydrophilic material, meaning it has a tendency to absorb moisture. If allowed to dry out, it can warp or shrink, which would affect the accuracy of the impression and ultimately the fit of any dental restorations made from it.

In contrast, addition silicone, polyether, and polyvinyl siloxane do not require a damp storage condition after use. These materials are less prone to distortion from moisture loss. Addition silicone and polyvinyl siloxane are more stable once set, and while polyether is also stable, it does not have the same moisture sensitivity as alginate. Therefore, understanding the properties of these materials is crucial for proper handling and ensuring high-quality dental impressions.