Which of the following is an example of a vat dye?

Step-by-step solution

Vat dyes are insoluble in water and cannot be applied directly to fabrics. They are first reduced to a colourless, water-soluble form in a large vessel called a vat, applied to the fabric, and then oxidised back to the insoluble coloured form. Indigo is a classic example of a vat dye. Alizarin is a mordant dye. Malachite green is a basic dye. Congo red is a direct dye. Answer: Indigo