The correct answer is A. Tri-calcium silicate.
Tri-calcium silicate (C3S) is the most abundant compound in Portland cement, making up about 60% of its weight. It is also the compound that reacts most quickly with water, setting the cement and giving it strength.
Di-calcium silicate (C2S) is the second most abundant compound in Portland cement, making up about 25% of its weight. It reacts more slowly with water than C3S, but it still contributes to the strength of the cement.
Tri-calcium aluminate (C3A) is the third most abundant compound in Portland cement, making up about 10% of its weight. It reacts very quickly with water, but it also forms ettringite, which is a gel that can cause the cement to expand and crack.
Tetra calcium alumino ferrite (C4AF) is the least abundant compound in Portland cement, making up about 5% of its weight. It reacts slowly with water and does not contribute significantly to the strength of the cement.
In summary, the compound of Portland cement which reacts immediately with water and also sets first is tri-calcium silicate (C3S).