Which one of the following statements is correct?

1. The question asks for the correct statement among the options related to chemical substances. 2. Option (A) The presence of NaCl increases the rate of setting of Plaster of Paris. This is generally incorrect. Accelerators for Plaster of Paris (calcium sulfate hemihydrate) setting are typically substances like potassium sulfate or gypsum itself. Chlorides like NaCl often act as retarders or have complex effects, not straightforward acceleration. 3. Option (B) Gypsum is added to cement to slow down its rate of setting. This is correct. Gypsum (calcium sulfate dihydrate) is added to Portland cement clinker during grinding to control the setting time, specifically to prevent flash setting caused by tricalcium aluminate (C3A). 4. Option (C) All alkaline earth metals form hydrated salts. This is generally true. Alkaline earth metals (Group 2: Be, Mg, Ca, Sr, Ba, Ra) readily form ions with a +2 charge. These ions have a high charge density (especially the smaller ones) and attract water molecules strongly, leading to the formation of hydrated salts (e.g., MgSO4·7H2O, CaCl2·6H2O). While the degree of hydration varies, the tendency to form hydrates is characteristic. 5. Option (D) Barium and Strontium are found free in nature. This is false. Barium and Strontium are highly reactive alkaline earth metals. Due to their reactivity, they are never found in their elemental (free) state in nature; they always occur combined in minerals (e.g., Barite - BaSO4, Celestite - SrSO4). 6. We have identified (B) and (C) as correct statements. However, the provided answer key is (D). This contradicts chemical facts, as (D) is definitively false. Let's re-evaluate. Perhaps 'found free' in (D) meant 'found easily'? Unlikely. Perhaps 'all' in (C) is too strong? Some salts might be anhydrous under certain conditions, but the tendency is strong. Is (B) always true? Yes, gypsum's role as a retarder is fundamental to cement chemistry. Is (A) completely false? Some sources might list NaCl as having complex effects, maybe accelerating certain phases. Given the clear correctness of (B) and incorrectness of (D), there is a high probability of an error in the provided answer key (D). *If forced to choose (D) as correct, the analysis would be baseless.* Assuming the key is wrong, (B) or (C) could be correct, with (B) being a very standard, well-known fact in cement chemistry.