Name one of the primary ingredients in hair relaxers.

Sodium hydroxide is a key ingredient in hair relaxers due to its strong alkaline properties, which enable it to break down the protein structure of the hair. When applied, it penetrates the hair shaft and alters its chemical bonds, ultimately relaxing curls or waves and making the hair straighter. This process is effective because it raises the pH level of the hair, causing the cuticle to open and allowing the relaxer to work on the cortex where the hair's texture is determined.

Other options such as calcium carbonate, magnesium sulfate, and pantothenic acid do not possess the same hair-altering chemical properties. Calcium carbonate is primarily used in various industries, including construction and as a dietary supplement, while magnesium sulfate is more commonly associated with uses like therapy or as a laxative. Pantothenic acid, on the other hand, is a vitamin (B5) that supports hair health but does not serve a function in chemically relaxing hair. Thus, sodium hydroxide is the clear choice when discussing the primary ingredient in hair relaxers.