The correct answer is A. Tri-calcium silicate.
Tri-calcium silicate is the most important constituent of Portland cement, accounting for about 60-70% of its weight. It is the main source of strength in cement, and it is responsible for the initial setting and hardening of concrete.
Di-calcium silicate is the second most important constituent of Portland cement, accounting for about 20-25% of its weight. It is also a source of strength in cement, but it is not as important as tri-calcium silicate.
Tri-calcium aluminate is the third most important constituent of Portland cement, accounting for about 5-10% of its weight. It is a source of strength in cement, but it is also the main source of heat in cement. This heat can cause problems with concrete, such as cracking and expansion.
Tetra calcium alumino ferrite is the least important constituent of Portland cement, accounting for about 1-3% of its weight. It is not a source of strength in cement, and it does not contribute to the setting or hardening of concrete.
In conclusion, good quality cement contains a higher percentage of tri-calcium silicate. This is because tri-calcium silicate is the main source of strength in cement, and it is responsible for the initial setting and hardening of concrete.