If all other factors are equal, what will probably lengthen lightening processing time?

The correct answer is that curlier hair will probably lengthen lightening processing time. This is due to the structural differences in curlier versus straighter hair. Curly hair tends to have a more complex structure with denser cuticle layers that can create a barrier to the lightener. This means that the chemicals in the lightening product must work harder and longer to penetrate the hair shaft effectively.

In contrast, finer and straight hair typically process lighteners more quickly due to their less dense structure. Coarse hair, while it may take longer to lift color due to its thickness, usually has a more open cuticle layer that allows lighteners to penetrate faster compared to curlier hair. Thus, the propensity for curlier hair to lengthen processing time is largely attributed to its intricate curl patterns and tighter cuticle layers, which can slow down the chemical action of lighteners.