Which of the following products is commonly used to straighten hair?

Sodium hydroxide is commonly used in hair straightening treatments, particularly in a type of chemical relaxer known as "lye relaxers." This chemical works by breaking down the protein structure in the hair, specifically the disulfide bonds that give hair its curl and wave. When applied to the hair, sodium hydroxide alters its texture and allows for straightening. It is favored in some treatments for its effectiveness on coarse, resistant hair types.

Other products listed, such as hydrogen peroxide, primarily serve different functions, like bleaching or color processing, rather than straightening. Ammonium thioglycolate is another chemical often used in perms and can be used for relaxing hair; however, its primary function is to re-configure curl patterns rather than straightening hair in the manner that sodium hydroxide does. Dimethicone is a silicone-based polymer used primarily for conditioning and smoothing hair, providing shine and reducing frizz, but it does not chemically alter the hair structure to achieve permanent straightening. Thus, sodium hydroxide stands out as the traditional and most effective choice for straightening hair in the context of chemical treatments.