The correct answer is C. CaSO4.2H2O, also known as gypsum. Gypsum is a common retarder for cement, and it is often used in concrete mixes to slow down the setting time. This can be helpful in hot weather, when
the concrete is more likely to set too quickly. Gypsum also helps to improve the workability of concrete, making it easier to place and finish.Option A, CaO, is calcium oxide, also known as quicklime. Quicklime is a strong alkali, and it can accelerate the setting of cement. It is not typically used as a retarder.
Option B, Al2O3, is aluminum oxide. Aluminum oxide is a very strong abrasive, and it can damage concrete. It is not typically used as a retarder.
Option D, Na2O + K2O, is sodium oxide and potassium oxide, also known as soda ash and potash. Soda ash and potash are alkalis, and they can accelerate the setting of cement. They are not typically used as retarders.