ANS:A - in the blast furnace shaft.

In the context of a blast furnace, the term "shaft" refers to the vertical portion of the furnace where the reduction of iron ore occurs. The blast furnace shaft is the central region where the raw materials, including iron ore, coke, and limestone, are charged from the top and gradually descend downward as they are subjected to high temperatures. The solution loss reaction primarily takes place in the upper part of the blast furnace shaft. In this region, the iron oxide in the iron ore reacts with carbon monoxide (CO) and other reducing agents produced from the combustion of coke. This reaction results in the reduction of iron oxide to metallic iron (Fe). However, as the iron oxide is reduced, it also dissolves into the molten slag, which is formed from the fluxes added to the furnace, such as limestone (calcium carbonate). This dissolution of iron into the slag constitutes the solution loss reaction. The solution loss reaction is essential for the extraction of iron from the iron ore in the blast furnace. It contributes to the formation of molten iron and slag, which are subsequently tapped from the furnace for further processing.