What major innovation occurred in hair care during the 1930s?

The correct answer highlights a pivotal moment in the evolution of hair care. During the 1930s, the development of heat-free perms marked a significant advancement in hairstyling technology. These methods allowed for the permanent reshaping of hair without the application of heat, which was a valuable alternative to the previous heat-dependent perm techniques. This innovation transformed the hairstyling experience, making it safer and more accessible to a broader range of clients.

The other innovations, while significant, emerged either before or after this pivotal decade, and did not share the same level of impact on the industry at that particular time. For instance, electric combs and hair dryers had already been introduced prior to the 1930s, and permanent hair color continued to evolve in various forms but did not see the same breakthrough that heat-free perms represented. Therefore, the recognition of heat-free perms as a major innovation during the 1930s underscores its historical significance in the progression of hair care practices.