The correct answer is D. Magnesium carbonate.
A mordant is a substance that is used to fix a dye to a fabric or leather. It does this by forming a chemical bond with the dye, which makes the dye more resistant to fading and washing. Magnesium carbonate is a common mordant because it is inexpensive and easy to use. It is also non-toxic and does not damage the fabric or leather.
Magnesium oxide, magnesium chloride, and magnesium sulfate are not used as mordants. Magnesium oxide is a white powder that is used in a variety of applications, including as a fire retardant, an abrasive, and a filler. Magnesium chloride is a white, water-soluble salt that is used in a variety of applications, including as a desiccant, a flux, and a fertilizer. Magnesium sulfate is a white, water-soluble salt that is used in a variety of applications, including as a fertilizer, a laxative, and a drying agent.