For a good quality cement, the ratio of silica to alumina is found to be

We need to identify the correct ratio of silica to alumina in good quality cement.

Portland cement is manufactured by heating limestone ($$CaCO_3$$) with clay containing $$SiO_2$$ and $$Al_2O_3$$ in a kiln, and its quality depends on the ratios of these oxides.

For good quality Portland cement, the ratio of silica ($$SiO_2$$) to alumina ($$Al_2O_3$$) should be approximately 2.5 to 4, with an ideal value of about $$3$$, while the ratio of lime ($$CaO$$) to the total acidic oxides ($$SiO_2 + Al_2O_3 + Fe_2O_3$$) is about $$2$$.

When the silica to alumina ratio is too low, the cement sets quickly but lacks strength; conversely, a ratio that is too high causes slow setting.

Hence, the correct answer is Option 4: 3.