What is the function of a lightener in a hair treatment?

A lightener in hair treatment is specifically formulated to alter the natural pigmentation of hair, effectively making it lighter. This process works by removing or modifying the color pigment within the hair shaft. Lighteners contain chemicals, such as hydrogen peroxide or ammonia, that facilitate this chemical reaction, allowing for a significant change in hair color, often achieving shades that are much lighter than the natural color.

In the context of hair treatments, options that refer to enhancing scalp circulation, adding shine, or nourishing damaged hair pertain to different products or treatments, such as scalp massagers, conditioners, or serums, which serve separate purposes focusing on health and aesthetics rather than changing color. Therefore, the primary and defining function of a lightener is to achieve a lighter hair color, making the identified option the correct choice.