Ammonium thioglycolate primarily causes the hair to:

Ammonium thioglycolate is a molecule commonly used in chemical hair treatments, particularly for perming and relaxing hair. Its primary function is to soften the hair, making it more pliable and easier to mold into different shapes.

When ammonium thioglycolate is applied to the hair, it breaks down the disulfide bonds that give hair its structure. This process effectively alters the hair's natural texture, allowing for the creation of curls or waves when the hair is then styled. Because it loosens the internal structure of the hair, it results in a softer feel and enhances the hair's ability to take on new shapes.

The other options such as strengthening, lengthening, and darkening involve different chemical processes and treatments that do not align with the function of ammonium thioglycolate. Strengthening typically requires protein treatments for reinforcement, while lengthening is associated with hair extensions or growth treatments. Darkening hair is achieved through dyes or color treatments rather than through the use of ammonium thioglycolate. Therefore, the primary action of ammonium thioglycolate is to soften the hair, which aligns with the correct answer.