Which type of hair has the smallest diameter?

Vellus hair is characterized by its fine texture and small diameter. It is the soft, thin hair commonly found on areas such as the face and arms, providing a thin layer of insulation and protection for the skin. This type of hair is not typically pigmented and is much lighter in color compared to terminal hair, which is thicker, coarser, and more pigmented.

Coarse hair refers to hair that has a larger diameter and is typically stronger and more resilient. Medium hair falls in between coarse and fine hair in terms of texture and thickness. Thus, when comparing the different types of hair strands, vellus hair is recognized as having the smallest diameter, making it the correct choice in this context. Understanding these distinctions is essential in cosmetology for hair care and treatment processes.