Final answer:
The mordant used in Gram staining to fix crystal violet to cells is iodine. This substance, known as Gram's iodine, complexes with the crystal violet, helping to retain the dye in Gram-positive cells, which remain purple, while Gram-negative cells appear pink after staining with safranin. The option (a) is correct.
Step-by-step explanation:
In the Gram staining procedure, the reagent that acts as a mordant to fix crystal violet to cells is iodine. After initially staining the cells with crystal violet dye, Gram's iodine is applied. This substance acts as a trapping agent that complexes with the crystal violet, forming a large crystal violet-iodine complex that clumps and remains within thick layers of peptidoglycan in the cell walls.
During the decolorization step of the procedure, ethanol or an acetone/ethanol solution is used to selectively remove the stain from only the Gram-negative cells, which possess thinner peptidoglycan layers. Finally, the secondary counterstain, usually safranin, is applied, staining the decolorized Gram-negative cells pink, while the Gram-positive cells remain purple. Therefore, option (a) is correct.