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

ANS:B - Gypsum

Gypsum will react with the tricalcium aluminate mineral (C3A) which is the compound mainly responsible for the early setting time of cement. The reaction of gypsum with (C3A) will retard the setting time of cement (i.e Prolong the setting time) which is necessary for concreting operations to be completed perfectly. The terms cement and concrete are used interchangeably, although cement is actually an ingredient of concrete. Gypsum is often added to cement and concrete to slow down the drying time. Cement dries quickly and retarding that process allows workers to pour the mixture into the various forms or frames needed. Gypsum retards initial setting, not initial setting time.
Gypsum prolongs the initial setting time!
There is the difference!