What strong substance is commonly used in permanent waves and chemical relaxers?

The correct answer is sodium, as it is commonly used in permanent waving and chemical relaxers primarily in the form of sodium hydroxide. Sodium hydroxide is a strong alkaline substance that can break down the disulfide bonds in the hair's structure, allowing for the reformation of these bonds into a new shape, whether that's achieving curls or straightening the hair.

In hair processing, especially with permanent waves, sodium hydroxide effectively allows the hair to be softened and molded into new styles. Similarly, in chemical relaxers, it serves a critical role in straightening the hair by effectively relaxing the natural curl pattern.

The other substances listed, such as potassium, calcium, and magnesium, do have their uses in various settings, including cosmetic and health applications, but they are not the primary agents used in the formulation of chemical texturizers like permanent waves and relaxers. Potassium can be found in other types of hair products but doesn't have the same strong alkaline properties as sodium hydroxide necessary for consistent and effective chemical processing in this context.