Which one of the following chemicals is used in Beauty Parlours for hair-setting?

Hair-setting in beauty parlours, particularly permanent waving (perming) and chemical relaxing, relies on altering the chemical structure of hair keratin. Hair strength and shape are maintained by disulfide bonds, which are covalent bonds joining sulfur atoms in adjacent polypeptide chains [t3, t8]. During the hair-setting process, a reducing agent—most commonly thioglycolic acid or ammonium thioglycolate—is applied to break these disulfide bonds by adding hydrogen atoms [t3, t4]. This allows the hair to be physically reshaped using rods or rollers. Once the desired shape is achieved, a neutralizer (typically an oxidizing agent like hydrogen peroxide) is applied to reform the sulfur-to-sulfur bonds in their new configuration, making the style permanent [t3, t4, t6]. Consequently, the chemicals used for hair-setting are fundamentally sulfur-based due to their interaction with the sulfur-rich cysteine amino acids in hair.