The major component/ingredient of Portland Cement is:

We begin by recalling the approximate composition of ordinary Portland cement, which is a carefully proportioned mixture of four chief crystalline compounds. These compounds, with their usual percentage ranges by mass, are

$$\text{Tricalcium silicate (C}_3\text{S)} \;=\; 3\text{CaO}\cdot \text{SiO}_2 \;\approx\; 50\% \text{ to } 60\%,$$

$$\text{Dicalcium silicate (C}_2\text{S)} \;=\; 2\text{CaO}\cdot \text{SiO}_2 \;\approx\; 20\% \text{ to } 30\%,$$

$$\text{Tricalcium aluminate (C}_3\text{A)} \;=\; 3\text{CaO}\cdot \text{Al}_2\text{O}_3 \;\approx\; 5\% \text{ to } 12\%,$$

$$\text{Tetracalcium aluminoferrite (C}_4\text{AF)} \;=\; 4\text{CaO}\cdot \text{Al}_2\text{O}_3\cdot\text{Fe}_2\text{O}_3 \;\approx\; 5\% \text{ to } 10\%.$$

Among these, the compound present in the highest proportion is clearly tricalcium silicate, written chemically as $$\text{Ca}_3\text{SiO}_5$$ and abbreviated in cement chemistry notation as $$\text{C}_3\text{S}.$$ Because its percentage is the greatest—roughly one-half to three-fifths of the total mass—it is called the major component or principal ingredient of Portland cement.

Looking now at the four options given:

A. dicalcium silicate  ($$\text{C}_2\text{S}$$)
B. tricalcium aluminate  ($$\text{C}_3\text{A}$$)
C. dicalcium aluminate  (“C2A”, which is not a principal listed phase)
D. tricalcium silicate  ($$\text{C}_3\text{S}$$)

Only option D matches the compound that we have just identified as being present in the largest proportion. Therefore the major component of Portland cement is tricalcium silicate.

Hence, the correct answer is Option D.