ANS:A - SiO₂

The acidic constituent of Basic Oxygen Furnace (B.F.) slag is SiO2 (Silicon dioxide). In the Basic Oxygen Furnace process, silicon dioxide (SiO2) is added to the furnace charge as fluxing material. During the steelmaking process, SiO2 reacts with impurities such as phosphorus and sulfur, forming slag. This slag is then removed from the molten steel and discarded as waste. Since SiO2 is an acidic oxide, it contributes to the acidic nature of the slag.