The correct option (a) slag
Explanation

In blast furnace, the lime stone is decomposed to give lime and carbon dioxide. The lime obtained act as flux and combines with silica to form calcium silicate (slag).

CaCo3 gives rise to Cao +Co2
limestone


Cao + Sio2 ( flux ) gives rise CaSio3 (slag)

The Metallurgy of Iron and the Role of Limestone in the Blast Furnace Process

Introduction
In the metallurgy of iron, the blast furnace process plays a crucial role in extracting iron from its ore, which is primarily hematite (Fe2O3). This process involves the reduction of iron ore with coke (carbon) in the presence of limestone (CaCO3). Limestone is added to the blast furnace as a fluxing agent, and it serves multiple purposes during the process.

The Function of Limestone
Limestone plays a vital role in the blast furnace process due to its chemical composition and properties. It acts as a fluxing agent, which means it helps to facilitate the removal of impurities from the iron ore. The addition of limestone to the blast furnace accomplishes the following:

1. Formation of Slag: When limestone is heated in the blast furnace, it decomposes to produce lime (CaO) and carbon dioxide (CO2). The lime reacts with impurities present in the iron ore, such as silica (SiO2), to form a liquid slag. The slag floats on top of the molten iron and helps in the removal of impurities. Therefore, the calcium ion (Ca2+) from limestone ends up in the slag.

2. Neutralization of Acidic Impurities: The limestone also helps neutralize the acidic impurities present in the iron ore. These acidic impurities, such as sulfur (S), phosphorus (P), and silicon (Si), can adversely affect the quality of iron. The calcium oxide (lime) produced from limestone reacts with these acidic impurities to form stable compounds, which are then removed as part of the slag.

3. Temperature Control: Limestone aids in maintaining the temperature inside the blast furnace. The decomposition of limestone to lime requires a significant amount of heat, which helps to sustain the high-temperature conditions necessary for the reduction of iron ore. This ensures the efficient operation of the blast furnace.

Conclusion
In summary, the addition of limestone to the blast furnace in the metallurgy of iron serves several important functions. It facilitates the formation of slag, which aids in the removal of impurities from the iron ore. Moreover, limestone helps neutralize acidic impurities and assists in temperature control within the furnace. Therefore, the calcium ion from limestone ultimately ends up in the slag, making option 'A' (Slag) the correct answer.