The correct answer is A. Gypsum.
Gypsum is the most commonly used retarder in cement. It is a white, powdery mineral that is added to cement to slow down the setting time. This allows for more time to place and finish the concrete before it hardens. Gypsum also helps to improve the workability of the concrete and reduce shrinkage.
Calcium chloride is a white, crystalline salt that is sometimes used as a retarder in cement. However, it is not as effective as gypsum and can cause the concrete to become more brittle. Calcium carbonate is a white, powdery mineral that is sometimes used as an accelerator in cement. It does not have any retarding properties.
Here is a table that summarizes the properties of each option:
| Option | Description |
|—|—|
| Gypsum | A white, powdery mineral that is added to cement to slow down the setting time. |
| Calcium chloride | A white, crystalline salt that is sometimes used as a retarder in cement. |
| Calcium carbonate | A white, powdery mineral that is sometimes used as an accelerator in cement. |