Which service requires the use of a relaxing or smoothing treatment?

The focus on using a relaxing or smoothing treatment is specifically necessary for straightening curly or frizzy hair. This service aims to alter the hair's texture and achieve a smoother, more manageable style. Relaxers and smoothing treatments work by breaking down the bonds in the hair’s structure, allowing for a change in its natural curl pattern.

The other choices, while relevant to hair services, do not specifically require such treatments. Enhancing curls involves products that may define and hold curl patterns rather than alter them. Coloring hair is about changing the hair color and does not inherently involve altering the texture. Layering hair is a cutting technique that shapes the hair for better movement and volume, which also does not necessitate the use of relaxing or smoothing treatments.