The correct answer is: B. Calcium chloride
Calcium chloride is a common admixture used to accelerate the initial set of concrete. It does this by increasing the rate of hydration of the cement, which is the process by which the cement particles react with water to form a solid matrix. Calcium chloride can also be used to reduce the bleeding of concrete, which is the tendency of water to rise to the surface of the concrete and form a film.
Gypsum is another common admixture used in concrete, but it does not have the same effect on the initial set of concrete as calcium chloride. Gypsum is used to control the setting time of concrete, and it can also be used to improve the workability of concrete.
Calcium carbonate is not typically used as an admixture in concrete. It is a common mineral that is found in limestone and chalk, and it is used in a variety of other applications, such as papermaking and ceramics.
In conclusion, the most common admixture which is used to accelerate the initial set of concrete is calcium chloride.