In the dyeing and tanning industry, a mordant is a substance used to help the dye adhere to the fabric or leather. The correct answer is option 4, Magnesium sulphate.

Magnesium sulphate, also known as Epsom salt, is commonly used as a mordant in the dyeing and tanning industry. It acts as a fixative by forming a chemical bond with the dye molecules, making them more resistant to fading and washing out. This helps to ensure that the color stays vibrant and lasts longer.

Option 1, Magnesium oxide, is not typically used as a mordant in dyeing and tanning. It is more commonly used as a refractory material, catalyst, or in the production of magnesium metal.

Option 2, Magnesium carbonate, is also not commonly used as a mordant in dyeing and tanning. It is primarily used in the pharmaceutical industry as an antacid and as an additive in some food and cosmetic products.

Option 3, Magnesium chloride, is not typically used as a mordant in dyeing and tanning either. It is more commonly used as a de-icing agent, in the production of magnesium metal, and in the preparation of tofu.

Therefore