The correct answer is B. 30 minutes.
The initial setting time of cement is the time it takes for the cement to start setting after mixing with water. It is measured by the Vicat needle test, in which a needle is inserted into the cement paste and the time it takes for the needle to penetrate a certain distance is recorded. The initial setting time should be at least 30 minutes for ordinary Portland cement, as this ensures that the cement has time to set properly before it is placed in the formwork.
Option A is incorrect because the initial setting time should be at least 30 minutes, not 10 minutes.
Option C is incorrect because the initial setting time should be at least 30 minutes, not 60 minutes.
Option D is incorrect because the initial setting time should be at least 30 minutes, not 600 minutes.