Which reagent is used for blueing a hematoxylin-stained section?

The correct choice refers to an alkaline solution, which is key in enhancing the color of hematoxylin-stained sections. When hematoxylin is used in staining, it typically produces a purple or blue color. To achieve a more distinct blue hue from hematoxylin, an alkaline environment is necessary. This is because the blue color develops as a result of the oxidation of hematoxylin in the presence of an alkaline solution.

In the context of histology, an alkaline solution often contains an agent such as ammonia or sodium bicarbonate, which raises the pH and facilitates the transformation of the color. The process, referred to as "blueing," improves the visual contrast of cell structures and can be essential for clearer observation under a microscope.

The other options do not promote blueing in the same effective manner. Acidic solutions can actually reverse the color change and lead to a pinkish hue rather than enhancing the stability of blue. Alcohol solutions do not contribute to the development of the blue color of hematoxylin, as they are primarily utilized for dehydration and not for alteration of the staining properties. Mordant solutions also have a different role, usually serving to fix or bind the dye to the tissue rather than enhancing color saturation.