To retard the initial setting time of cement, the compound responsible, is

Cement setting is a chemical reaction that causes it to harden. To control this process and prevent it from setting too quickly, we add certain compounds.

Gypsum (calcium sulfate) is specifically added to cement to retard, or slow down, the initial setting time. This ensures that the cement has enough time to be mixed and placed before it hardens. The other options listed (tri-calcium silicate, di-calcium silicate, and tri-calcium aluminate) are all components of cement itself, but they do not primarily control the initial setting time in the same way gypsum does.

Therefore, the correct answer is B: Gypsum.