Final answer:
The main ingredient in a cold or alkaline perm is ammonium thioglycolate, which is used to break and re-form disulphide bonds in hair, permitting the hair to be reshaped into curls or waves. Option a is correct.
Step-by-step explanation:
The main ingredient of a cold perm, which is also known as an alkaline perm, is ammonium thioglycolate. This chemical is responsible for breaking the disulphide bonds in hair keratin, which allows the hair to take on a new shape when it is wrapped around perm rods.
Once the hair has been reshaped, a neutralizer, typically hydrogen peroxide, is applied to reform the disulphide bonds in their new configuration, thus setting the hair in its new curly or wavy form. In comparison, sodium hydroxide is mainly used for chemical straightening or relaxing, but not for perms.