ANS:A - form slag by combining with impurities.

Limestone is added in the blast furnace during pig iron manufacture to: "Form slag by combining with impurities." The addition of limestone (calcium carbonate, CaCO3) to the blast furnace serves to flux the impurities present in the iron ore. When limestone is heated in the blast furnace, it decomposes to form lime (calcium oxide, CaO) and carbon dioxide (CO2). The lime then reacts with impurities such as silica (SiO2) and alumina (Al2O3) present in the iron ore to form a molten slag. This slag floats on top of the molten iron and helps separate impurities from the iron. Therefore, limestone plays a crucial role in forming slag by combining with impurities in the blast furnace.