Concept: A mordant is a substance that helps fix a dye onto a fabric or material.Explanation:Aluminium hydroxide, Al(OH)3, is not the colouring agent itself.It acts as a binding agent between the dye and the textile fibres.This binding makes the dye stick firmly and prevents it from washing away easily.Therefore, in the dyeing industry, aluminium hydroxide is used as a mordant.Answer: D. mordant