Which of the following is NOT a hair replacement technique?

Cosmetic hair dyes are primarily used to alter the color of existing hair rather than to replace or restore hair that is missing or thinning. This technique does not address hair loss directly nor does it provide any means of regrowing follicles or replacing hair strands. In contrast, the other options—hair solutions, surgical hair transplants, and scalp reduction—specifically focus on addressing hair loss through various methods whether they be temporary, permanent, or surgical in nature.

Hair solutions can include a range of options like wigs, hairpieces, or topical applications that assist in making the hair appear fuller or to cover bald spots. Surgical hair transplants involve the actual transplantation of hair follicles from one part of the scalp to another, making it a direct method of hair replacement. Scalp reduction is a surgical procedure that removes balding scalp areas and stretches the hair-bearing scalp to compensate for the loss, thus fitting into the category of hair replacement techniques.

In summary, while cosmetic hair dyes enhance and change hair color, they do not replace hair, making them the correct choice for an answer to the question about which option is not a hair replacement technique.