Final answer:
The reagent in a Gram stain that acts as a mordant is iodine (Option. B). It forms a complex with crystal violet, helping Gram-positive bacteria retain the purple color after decolorization, while Gram-negative bacteria will be decolorized and subsequently counterstained pink by safranin.
Step-by-step explanation:
The reagent in a Gram stain that acts as a mordant to bind the stain to the bacteria is iodine (B). A mordant is a substance that increases the affinity of a stain by forming a complex with the dye, which then binds to the cellular components. In the Gram stain procedure, after applying the primary stain, crystal violet, Gram's iodine is added. It interacts with crystal violet to form a larger complex that is trapped within the thick peptidoglycan layer of Gram-positive bacteria, helping retain the purple color after decolorization.
The decolorizing agent, usually ethanol or an acetone/ethanol solution, is then used to remove the purple stain from Gram-negative bacteria, which have a thinner peptidoglycan layer and cannot retain the complex. Lastly, safranin is used as a counterstain to provide a pink color to the now colorless Gram-negative bacteria, while the Gram-positive bacteria remain purple due to the retention of the crystal violet-iodine complex.