Option 1: Resin is not commonly used in the leather industry. Resin is mostly used in the production of plastics, varnishes, and adhesives.

Option 2: Mucilage is also not commonly used in the leather industry. Mucilage is typically used as a thickening agent in food products and as a binder in pharmaceuticals.

Option 3: Latex is not directly used in the leather industry. Latex is primarily used in the production of rubber products, such as gloves, balloons, and tires.

Option 4: Tannin is the correct answer. Tannin is a substance found in plants, especially in the bark and leaves, that has a strong astringent effect. It is commonly used in the leather industry for tanning animal hides and turning them into leather. Tannin helps in preserving and strengthening the leather.

So, the correct answer is option 4, Tannin.