What does a strand test determine when coloring hair?

A strand test is a crucial procedure conducted prior to coloring hair, and it primarily serves to assess how the hair will react to the chosen dye. This involves applying the color to a small section of hair, which allows the stylist to observe the results before applying the dye to the entire head. By doing this, the stylist can determine the processing time needed for the desired outcome, as hair may respond differently based on its unique characteristics, such as porosity, texture, and existing color.

This test helps ensure that the color will achieve the intended result while minimizing potential unwanted reactions, such as breakage or a color result that is too dark or light. It also aids in adjusting the formulation or processing time accordingly, making it an essential step in the hair coloring process.

The other choices do not directly pertain to the primary function of the strand test. For instance, assessing overall hair health is important but does not reflect the immediate purpose of a strand test. Similarly, selecting a haircut style and determining hair thickness relate more to hair design and structure rather than the coloring process. Thus, option C accurately captures the intent and usefulness of conducting a strand test in the context of hair coloring.