The correct answer is A. Tri-calcium silicate.
Tri-calcium silicate is the main component of Portland cement, which is the most common type of cement used in construction. It is a hard, brittle material that reacts with water to form a strong cement paste. Tri-calcium silicate is also the main component of rapid hardening cement, which is a type of cement that sets and hardens more quickly than regular Portland cement.
Tri-calcium aluminate is a minor component of Portland cement. It is a reactive material that can cause the cement to set and harden too quickly, which can lead to cracking and other problems. Tetra-calcium alumino-ferrite is also a minor component of Portland cement. It is a less reactive material than tri-calcium aluminate, but it can still cause problems if it is present in too high of a concentration. Di-calcium silicate is a minor component of Portland cement. It is a less reactive material than tri-calcium silicate and tetra-calcium alumino-ferrite, and it does not cause the same problems.
In conclusion, the correct answer is A. Tri-calcium silicate.